Odd that it worked fine for the first batch of ammo and issues with the next. Is this the white box, yellow box or paper wrapped ammo?
I'd try buying some other magazine to verify as others pointed out.
A few things you can do in the meantime:
- Grab one of the rounds, can you easily push the bullet into the case by pressing it into a firm object?
- If you reload, pull a few of the bullets and measure the diameter using a caliper. Are they 0.224" or undersized?
- Remove the magazine catch from the rifle. Inspect it that to ensure it does not have any burrs on the catch that could be restricting the magazine from seating properly.
- Check the feed ramps like in the example below:
Or that they are not misaligned or have a bad barrel extension like below:
Other possible causes could be that the barrel extension's feed ramps are too narrow or too steeply angled or possibly has burrs/tool markings.
Hopefully it's just a mag issue because that would be the easiest solution.