Have you taken the mag apart and cleaned it? Magazines can collect a lot of dirt, especially if you're shooting IPSC and are dropping them in the dirt! Pull the spring out and mop the inside well with a silicone cloth. No oily lubes here, it has to stay dry. Clean the follower too and try it again. Or then it could be a mag that someone kept loaded to the max and the spring has lost some tension.
Another possibility is that the recoil spring has been replaced with a heavier spring for heavier loads and now if you're back to lighter loads, the slide is not moving fully open to pick up another round (consistantly).