Make sure that all parts of the bolt carrier making contact with the upper receiver are properly lubed to reduce friction.
Check the OAL of your rounds. 2.260 is the OAL of M855 ammo, and if you have anything much different than that, it could cause problems. Shorter rounds tend to be more finicky in my experience.
If there's a lot of play between the mag and the receiver when the mag is seated, you can tighten it up a little by adjusting the mag catch. Take a pointy object and push in on the mag release button and rotate the mag catch to tighten or loosen as needed. Don't make it so tight you can't release the mag though.
Magazine issues are likely the culprit, and while G.I. mags are fine, you've got to test them first to make sure they feed, and if they don't, junk them. The usual mag issues:
-deformed feed lips
-poorly tensioned mag springs
There are some new mag followers that have been recently developed for the G.I. mags that work better than the old ones, however, you'll have to dig around to find them.
If the mags don't seem to be the issue, the next thing I would look at is installing a new buffer spring, and if THAT don't do it, than you may have some internal design spec issues that may need a smith to fix.
88 day wait for the state to approve my constitutional right to bear arms...