I have a fiberglass shoer stall that leaks water from under the stall but it only leaks when someone is in the shower it does not leak if you turn on the shower and stand outside, Oh it is installed on a raised platform on top of concrete. Thanks John

My bet's on the seal of the drain lip. (see image) If you still have a leak when you close off the drain and put a inch or so of water in the base then the first thing I would do is open up the ceiling, loosen the large retaining nut on the drain and run a small rope of plumbers putty under the drain lip. Now tighten the nut up, remove the excess putty that squeezed out and test again. Bet the leak's gone now. Good luck, Tom