Springs usually don't weaken from lack of use. Repeated use is what weakens springs.
A majority of the time that your situation occurs (as well as the slide NOT staying open after the last shot), one of the shooter's thumbs is nudging against the slide lock lever. When shooting, it doesn't take much force to nudge the lever up (or down)
Everyone swears that their thumbs are nowhere near the lever, but the next time this happens, **Freeze** and without moving your hands, check your grip. You may find that one of your thumbs is against the lever. Been there... Done that!