Quoted:
So after years and years of wanting one I finally got myself a 92fs. First trip to the range and I was very impressed with the feel and accuracy, especially with a fairly novice shooter (myself) behind the trigger. The only problem I was having was the slide almost never locked back on the last round. It seemed to do this with both factory mags. After a bit of searching on other forums it seems my grip may be to blame, at least partially. It seems that my grip puts my right thumb just over the top of the slide release lever and even the slightest amount of downward pressure will cause it not to lock back. Now on to the actual question, would getting some +10% wolff magazine springs solve the issue? Or can I just go ahead and get a couple mec-gar 18rnd mags and solve the issue that way? Or should I change my grip that I've used on every other previous pistol with no issue. Including another Beretta, a px4.

EDIT: DOH! I guess its worth mentioning that I was shooting mostly WWB 115grain rounds. Just a few rounds of sellier & bellot.

Quoted:
The shooting grip you are using is a very good grip, but it does have the problem of depressing the slide lock and not  allowing it to do its job.
Get more practice throwing rounds downrange and try to keep your thumb slightly off of the pistol controls. It can be overcome with practice.
Going to a heavy magazine spring is not going to help.