I had two mags I order from Bushmaster that were out of spec (Blued). These were crap. Some BM mags are ok, some are not. Sent them back for a refund and bought 2 new USGI mags which worked fine.
Stick with USGI mags and you will be fine.
tips1. When you load the mag, look for the cartridge shape molded into the follower - this tells you which way the cartridges need to be inserted into the mag.
2. Load your cartridges, then rap the back and top end of the mag sharply against the bench or your boot heel, this seats the cartridges to the back of the mag where they need to be to feed smoothly.
3. lock the bolt to rear, insert the magazine until you hear and feel the mag catch lock into place, you can give it a light slap on the base of the mag if you want. the cartridges should be pointed towards the front of the rifle.
4. Hit the bolt release, let the bolt slam forward, this will strip a cartridge from the mag, and chamber it, locking the bolt into battery. DO NOT ride the bolt forward with the charging handle.
5. Give your forward assist 2 firm taps with the heel of your right hand.
6. Ensure the weapon is on "fire", and engage your targets.
7. Savor the light recoil, the heady aroma of XM193 going off (you did buy XM193, didn't you
), the delightful "sproing" noise of your buffer at work, and the sublime joy of sending copper and lead downrange at several thousand feet per second.
IF you follow this procedure every time, you will be good. If it still doesn't work, there is something wrong with either the magazine or the weapon.