1. The mag catch may be looser than it was, and allowing the mag to tilt more in the mag well. As pointed out, try tightening the catch.
2.The mag well may be more worn that it was, and this is allowing the mag to tilt more as the rifle is being fired. Tightening the mag catch may help with this one.
3. The mag catch it's self may have worn, and this is allowing the mag to sag down/seat lower in the well more than before. Might want to try another mag catch.
4. If the rifle on the verge of short stroking, the buffer impacting off the back of the receiver may be less than is was before, and this less of impact/jolt is missing to allow the mag to recover (get the top round all the way back up to the feed lips as the bolt strips the round. Since I tend to be a buffer whore (read I run about every buffer thrown my way), my solution is to spring cant tweak all of my mags springs to make sure even if I don't get a strong impact of the buffer off the back of the receiver extension, the mag is going to get the top round up against the lips/ in the right angle as soon as the carrier clears the top round on the way back.
5. Using secret magic, there is a mag on the market the will allow even the funkiest round tip to be feed cleanly into the feed lips (read even the most unforgiving HP tips). If you have been following some of the threads over the last few years, you will remember that this is one mag that has had problems with the feed lips rubbing on the bottom of a great deal of receivers in the AR rifles since it is the highest ridding mag known. The trick will be trying to get your hands on the SA80 mags since the supply has seemed to dry up.