Are you sure this is a M88 magazine and not a M100?
Unload the magazine and look at the follower from the top. Does it say M100 or M88?
Sometimes the spring looses it's ability to spring back. If you remove the second round will the rest of the rounds in the magazine cycle?
Oh, the front of the magazine should have .243 and .308 marked on it.
That does not look like a factory magazine... what does it look like from the bottom?