hello, sorry your having trouble, but you already have the answer. you neve neve tile over tile. or you will be experience the problems over and over.your bath room floor is most likely a small area, rent a chipping hammer and take it all out even the thinset.besure to remove your toilet first. If you insist on not removing the tiles then take out the rest that are broken with a chisel and hammer then clean area well and re lay tile with a mulitpurpose thinset(mud)mortar. then keep doing this everytime one pos out. doesn’t this seem to be wasting your time? just do it right this time and you will not have to continual fix the problem. you have a concrete slab or wood? if it is wood you need to ist lay down durrock or an underlayment.