It sounds like you have a major problem with the 3/4 ply was not screwed down enough or the mud bed is not thick enough or if but may not be in your case but not to be over looked, if the floor was not covered with tar paper the concret can eat the screws and wood away. this could be some of the problem. Any way you should be all over the builder to fix this NOW. they should not naver installed any thing that could not with stand the test of time (house settling). I would be looking for some ones throt becouse he sounds like a smuck and will do this to some one else. Any way he is insured for this kind of thing.