I may have a couple of ideas.
First, make sure your mags are clean on the insides and that your mag springs are not installed backwards (makes heap big difference).
Second, take a peek at your magazine release button. On the inside of the mag well, you should be able to see the little catch/nub that actually holds the magazine body in the well. It should be sqaured. As a Beretta Armorer for my dept, we've found that not all of those mag catches are the same hardness and some wear out faster than others. Although I cannot rule out that the premature wearing of some catches are not a result of the shooters' habits and techniques of slamming the mags home. Anyway, if the mag catch is worn, it won't allow the magazine to seat as high as when it was factory new. Thus, if the mag isn't seated high enough then the 'slide lock shelf' on the follower can't assert it's upward force on the slide lock lever after the last round is discharged. I haven't looked for the complete magazine release button assemblies at Brownells but they have somewhat recently become a stocking factory part supplier and have most parts. Changing them out can be tedious and frustrating sometimes (esp if you have a magazine disconnect safety feature such as found on the 92D models). If you suspect that is the problem- drop me a line and I'll try to talk you through it. Also, there are many more superbly trained and skilled actual Gun Smiths here that I'm sure can help you as well.
Thirdly. Oh snap, I forgot what I was going to sugges... Oh yeah, it's not jsut real common, but it could be a problem with a weak mag spring or a follower that has seen much use and abuse or overly aggressive chemicals.
Just for poops and giggles, try firing one handed and use your off hand to push up firmly on the mag floor plate as you fire the last round from the mag. If that makes the slide lock to the rear- that is a good indicator that the mag release button is worn past it's usefulness.