A plumber has just tiled my bathroom, it looks great except the grouting between the floor tiles keeps crumbling. He tells me that it could be because of the porcelain floor tiles being layed onto a wooden floor. He has attempted again using a plastisiser in the mix but it has happened again. I know he layed a sub floor on top of the old wooden floor boards, (4ply marine Ply I think). Can anyone tell me a way to resolve this problem or should I just rip them all up and replace them with another type of tile??