Depends.
Might be an oxidized spring in the frame's spring hole. (Cleaning the spring and the spring hole is the usual solution.)
Might be a rough spot on the mag catch body and/or nut (bottom) as it moves in the recess. Believe it or not, after the parts are removed and cleaned and the mag catch button is adjusted (more on that in a moment), armorers are taught to remove any "roughness" in the motion of the mag catch button's movement ... by tapping the mag catch button with a plastic mallet to "set" the parts.
The mag catch is adjusted by turning the nut (button) half a turn at a time until the top of the small plunger is flush with the frame, or slightly below.
Every once in a while a particular gun (frame) might simply require a different mag catch assembly (body & nut), or a new spring (damaged, badly rusted, etc).
The plungers are rather small and are easily launched and lost.
(Wear eye protection
)
A "spanner" screwdriver is best for turning the nut (can be made by filing out the middle of a suitably wide slot screwdriver), but it can be done with regular tools. Getting the nut started on the threads of the mag catch body - while holding the plunger with the tip of a thin pin punch - is the tricky part.
The nut has slots at 180 degrees, and the top of the plunger is captured within one of the slots (hence, the half a turn at a time for adjustment.)
I've never tried the PC mag catch button, but then most of the duty & off-duty/plainclothes holsters I've used probably wouldn't have allowed sufficient clearance for the extended mag catch button, anyway. Having holsters flex with your movement (or simply being pressed against your body), and releasing the magazine - without you being aware of it - is usually somewhat annoying.
The "Hey, look, I've found a loaded magazine on the ground!" (or in the chair, your car seat, etc) can make for one of those memorable moments ... once you realize you're missing one.
