Most likely cause is that the sub flooring (the flooring material under the floor) is not stiff enough and/ or is not properly secured, or the structure supporting the sub flooring is not stiff enough.
Other possible causes include an incorrect and/or incorrectly installed type of adhesive material holding the tiles to the floor or a leak from a source such as a dishwasher, refrigerator icemaker or sink which is causing water to wick into the sub flooring.
The only way to diagnose the cause is for somebody who understands how the tile should be installed to inspect the current installation - if there is access to the floor from underneath they may be able to determine the cause from below, otherwise it will likely be necessary to remove tiles to diagnose the problem. (Sometimes, you can determine the cause by examining exposed sub flooring, tile, and adhesive under a dishwasher or base cabinet).
When there is extensive failure of a ceramic tile floor covering for any of these reasons, it is usually necessary to remove the tile, correcting the underlying cause of the problem, and then reinstall new tile to effect a permanent repair.
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