Define fit.
Loaded 10/30's do not like going in on a closed bolt on an AR15, they require a hard hit to seat the mag. A good method I was taught was push/pull (insert mag, push it in, pull to confirm it is locked in. If it comes out - it is not in, hit it hard to seat the mag).
Try seating on open bolt, loading on a closed bolt with only 9/10 rounds in the mags, and screaming at your mags to work well to trouble shoot/provide more info.