What happens is over time the foam base under the shower floor disintegrates. It flexes and cracks. You can buy a plastic repair kit at an auto parts store and repair the crack, reinstalling the floor over a better base that the original foam. Many others have done this, but if I were faced with the problem, I would first look to find a new pan. or perhaps as suggested above a new stall. Only problem I see with a one piece stall is getting it in place. Possible a four piece, three walls and a floor would be better.
Thor Four Winds 22e