I've heard all the stories, but... I wanted to know if any of you guys have some of the same, or if I just got a crap mag that's crap in new and exciting ways.
I own a few 20s I picked up for next to nothing when a local gunstore closed and those have actually been pretty reliable, so I figured I'd try my luck and picked a 30rd a while back for a C7 inspired build, just for the aesthetics really but well...

Anyway, the main problem I have is this one absolutely does not like to lock open. Even just cycling it by hand.
Unless the follower is in exactly the right position the bolt catch just does not engage.

When it does (or I've inserted it with the bolt open) it takes noticeably more force to use the bolt release than on any of my other mags.

This is more out of curiosity than anything, it's a relatively cheap mag that, again, I bought for aesthetic reasons more than anything, but I hadn't heard of them sucking in this way when I bought it and while not essential, it would be nice to be able to use it.
Is this something you guys have experienced, or does this sound like a QC issue?

Yes I did. I'll make that edit in a bit.

Like I said, no feeding issues with mine (well, ones that weren't the fault of the rifle), just other annoying issues.

Do they have Maple leafs on them?

No on the leaf. Just cheap American ones.

If NC made.
Contact them they will replace them.
Had some from the 80's during the ban that worked flawlessly.
Only issues I ever had with any Thermold magazines where Canadian 1's.

Thanks. I don't have that mag handy at the moment to confirm manufacturer location.
I did find a local place selling some old LE/GVT marked ones for a not soul crushing price.
I might pick one or two of them up and see if they have the same issues or if they were made to a better standard.

I have at least three Thermold 30 round mags from the early 90’s and they have been gtg.  I bought a 20 round cheap a couple years ago but I can’t remember if I’ve used it.

The only problem I've had with the Thermolds (preban ones), has been that some bolts hang up on the feed lips, and you have to cycle them many times to shave off some feed lip material, so the bolt will slip over them.  These are the 30rders. btw.

I was just going to ask if those were the firestarter stripper clips.
I remember in the late 80's...ish
People selling them as fire starters at gun shows claiming they burn for like 10 minutes.

They work very well for starting fires.

The one and only one I ever bought was a 10 rounder made in NC.  Intended it for bench use. It was crap.

The spring was just too weak to feed once it got down to the last 3 rounds.  Then it would not push the round up strongly enough for the round to be picked up and chambered.  Very wimpy.  It's sitting on s shelf somewhere.  Probably ought to see how many holes I can put in it as close together as possible at 100 yards.  Not worth sending one back for replacement, nor do I have any confidence in them.

See if it would work as a firestarter too, after you shoot it.

When the AW ban hit in 1994 I was not deeply stocked on AR mags, but I picked up a pretty good stock of Thermold preban 30 rounders where I paid a not-completely-crazy premium at a gunshow just after the ban where a dealer had several large boxes of them new; the Thermolds were actually fairly expensive before the ban and cost more than NIW USGI mags at the time (and back then USGI mags typically came in the same sealed clear plastic bags with mfg and CAGE code printed on them as the mil contract mags).  People seemed to focus on getting higher capacity 30s, so I later found decent prices on half a dozen Thermold 20 rounders I used for shooting prone with scoped rifles.  To prove out specific mags with higher round counts I put 6 of the 30s into rotation for regular shooting and 3 of the 20s and never had a single problem running Winchester USA branded 55gr (which I believe was better quality ammo back then, shooting 1" groups out of one 20" stainless heavy barrel) and IMI 55gr and 62gr.  (I should have dug out those remaining new 30s and put them on the EE years ago when that could have helped more people in preban-only states.)

I was aware of the Canadian military issue mags with the maple leaf molded into the side of the magazine and picked up 3 or so here and there at gunshows in various used condition from good to excellent. They had frequent failures to feed and I immediately designated then M16 historical collector's items only.  The Canadian issue are a much softer and more flexible plastic and I do not think the feed lips maintain geometry under spring pressure on a loaded magazine.  Pure speculation on my part, but I suspect the Canadians went to a compound that would be less brittle than the original compound in their extreme cold regions and seasons based on material property data sheets, but it was a huge reliability fail due to inadequate rigidity.  I think Thermold knew what they were doing with the original plastic compound, and it was the best 30 round AR magazine outside of USGI until Magpul came along years later and perfected the Pmag.

I also picked up a few new Orlites, one mag was iffy in one rifle, others ran OK, but I never ran them much after that preferring to reinforce the reliability of those half dozen 30 round Thermolds + another half dozen USGI 30 round mags I had by focusing round count on those. As I recall a lot of Orlites had the ridges machined off on each side that lined up with the bottom edge of the magazine well.  This was supposed to eliminate over insertion but with mag well variations it prevented them from seating and latching in many rifles.

I got a few HK91 Thermolds when HK factory mags where a fortune, but then the floodgates opened on cheap surplus G3 mags so I never ended up running them.

Cheap shit mags had  2 30s and a 20 that wouldn't work for shit. Springs seemed weak.

A few years ago I bought 50 of the 20rd US made ones when Numerich was selling the kits(complete mag but you had to assemble it yourself)for like $2/each. I checked every one of them in my rifle and they all fed fine but some fit a little tight in the magwell and wouldn't drop free. I put two of them into rotation and ran a few mags worth of ammo through them without any feeding issues. I prefer the 30rd mags and ended up selling the thermolds during one of the panics for three times what I paid.

I have a number of the old H22 and H23 batch US-made, and they work VERY well. I did have to clean up a little molding flash (especially around the back notch of the magazine. Once that was done, perfect. I have replaced the magazine springs with Wolff + springs, as these magazines got me through the AW ban.

A lot of Israeli surplus ones hit here after the AWB passed. some came with sand in them. They were cheap and most of them worked. I got rid of all of them after 2004.