I would suggest using a table knife to lift the tile ever so slightly to allow room for a small artist's paint brush to apply just enough tile glue and setting something heavy on top of the tile over night to give the glue a chance to grab. You may want to mask off the surrounding area to protect the floor from any over abundance of glue that may seep out.
This is ceramic tile that is a mud job...the tiles are concreted down.
From a carpenter retired after 50 years: 12" square tiles originated, probably, in Cuba or Spain. They never were flat and square, like we crazy Americans demand. You may be a victim of an incompetent tile person - but some beautiful house floors look that way on purpose. If you own one of them - celebrate!