Water leaking from a shower through downstairs ceiling

This is a recurrent issue. The leak is from a guest shower room which has been used infrequently. The following has already been done, but still water leaks through to and through the ceiling: 1. All joints between the tiling and the shower tray have been sealed with clear sealant. 2. The plug linking to the waste pipe has been tightened and dealed with bathroom sealant. There may be some other relatively simple solution, but I cannot think of how to resolve the problem. My thought would be that the only way to get at it would be to cut into the downstairs ceiling and fix whatever is the problem from underneath, The showere has a fully tiled floor except where the shower tray is. I therefore do not know if I need a plumber, a plasterer or a general handyman to fix the problem one more time. Your help would be appreciated.