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Sounds like there is not enough thickness of subflooring.You need a minimum of 1 1/8 inches(more is better) of subflooring and underlayment,It can be all wood but preferably 3/4 inch subfloor with 1/2 inch backer board (I.E. Durock or Hardibacker).
Pull up a floor vent for the heating and see if you can determine total thickness of what the ceramic is glued to.
If it is less than 1 1/8 inches then all tile should be removed ,proper backer installed then retiled.
Subfloor flexing is tiles worst enemy and is a very common cause for failure.
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