A Maintenance Issue

I was working with a guy the other day shooting a Smith & Wesson M&P40. several times the last round in the magazine failed to feed. The magazine was fully seated and when the shooter smacked the bottom of the magazine the slide snapped forward feeding the round. This took place with all his magazines. I broke it down and examined the weapon. I compared it to another M&P40 and the only difference between the two was that the trigger spring was bottom side up. I corrected that and test fired the gun with three magazines loaed with five rounds each. No problem. I then put three full magazines through it witht he same result. No problems. Any ideas, and yes I've considered the problem may be the shooter not the gun.