G&G F2000 (G2010) mid-cap feeding issue

[Grif 2200]

I've got an F2000 by G&G, I always use mid-caps and have done since i bought it. It used to feed perfectly with mid and high caps.

I took off the upper reciever to swap out the handguard from the stock ergonomic grip to the lower tactical rail, and ever since the silver bolt/pin that secures the upper reciever won't stay in position and keeps sliding out, but also doesn't feed from mid-caps very well. It feeds intermittently even if turned upside down. Bought brand new mid-caps, thinking the springs in the old ones were getting old, but to no avail. Seems to feed well-ish with high-caps but it's never guaranteed.

It only seems to have the issue with mid-caps, but only since I pulled off the upper reciever the first time to replace the handguard. Slightly less pressing, but nonetheless irritating, is the fact the bolt (Or pin/button, I'm not sure how best to describe it) no longer stays in place without duct-tape assistance.

Any advice for fixing this feeding issue, or sorting out this pin business? If possible, I'd rather stick to mid-caps, but it's just odd that even after having the tech at my local site take a look at it a few times the feeding isn't yet fixed. Maybe an owner of the G2010 who had the same issue has some wise words.

I've already shaved off some of the mags on top, to fit in and secure firmly in the magwell, which is the usual remedy according to the internet

Thanks in advance