So my Upper to Lower ratio has remained at a fairly steady 1:1.  And the uppers stay on whatever lowers I bought/built for 'em at the time.

Ended up building a new upper recently to replace the upper on an all-PSA rifle (wanted to go from 1:7 twist to 1:9, due to a combination of circumstances ended up building a whole upper vs. r&r barrel).

The take down pin won't re-insert all the way.  From what my aged eyes can tell, it gets through the hole in the tab/tang on the upper, but interferes with the lower on the inside just as it hits the exit hole.  

Went back, and tried all 4 of my uppers (2 PSA factory builds, one Spartan/Gibbz by me, one Anderson by me) on my 3 lowers (all PSA - one factory built, one by me and a PSA lpk, one by me and a JoeBobOutfitters LPK) in all of the combos.  Everything worked except the new upper (Anderson) on the one all-PSA factory assembled lower.  The other 3 uppers going onto this lower the take down pin is no where near as smooth for reinserting as any of the uppers on the other two lowers and to pop it out requires the use of a tool/bullet/etc. to poke - just getting it flush on the left side it is too tight to remove by fingers only.

Re-greased the take down pin detent and channel.  Checked spring for binding, etc. Looking at the take down pin it is rather scuffed up compared to the take down pin on a lower of similar age and use.

Figure the cause is just tolerance stacking - a lower made on a Monday at 8am and a upper made Friday at 5pm, etc.  

Planning on simply replacing take down pin, spring, and detent.  Parts are on the way.

Anything else that I should check/do ?

Stock tube screwed in past the inside top edge of the lower.

Maybe swap the pin out to make sure it is not the pin.

Got new parts in the mail today.  Just replacing the detent and pin didn't do it - in fact, with the old spring and new detent and new pin, I couldn't get it to release from either full open or full closed without an upper installed.  Figured it was the spring, and since I have a few spares on hand I got out the snips and started trimming.

Weird, but all fixed now.  

Also considering doing the "tap the hole and install a screw" mod to all my lowers now....