Hate to touch this, but a 20 round magazine that's got a greenie follower in it just ain't right.
In fact this is a indicator that someone is doing things to mags they should not be doing.
The most reliable mags out there are USGI 20 magazines and they all have alloy (aluminum lookin?) followers in 'em. Ya can't get better than those and even the ones I have that are 3 decades old still function 100% of the time.
The most reliable 30 rounders out there are again USGI or the LaBelles/DPMS ones, which in are the same as USGI, just sold under a different marketing name, IMO.
I never fully load my stored mags, I place 18 in the 20s and 28 in the 30s and I have never had a cycling problem. (of course this is not necessary as I load 20 or 30 in mags on occassion and they work fine, but for storage, I download 'cus I want to KNOW my mags will work when needed, this is a personal preference and something I do, 'cus they may be setting for years, YMMV)
FWIW, I have 10 of the brit SA80 mags, bought new from ammoman and they all work 100%, (note I consider these range mags tho), sounds as if you've identified your problem, get some new mags and stick to the USGI, till ya get 50 or so of the 30s and 20 or so of the 20s, (if ya like the 20s, I do and have way more than 20, YMMV)
I highly recommend getting 10-20 of the LaBelles sold on the EE by several, or the DPMS ones they are brand new and work well, but do both yourself and the mags a favor and if stored for years download 'em to 28, and they'll work flawlessly when used years from now.....
Mike