well, get the thinset off or buy new tiles. Scrape the floor area clean also. You want the new tile to be level with the contingent tile.
I would speculate that the tiler maybe did not get a good mastic coating on the 10 in question tile. There could have been a bubble or sorts under this area. Regardless, the adhesive did not take.
Butter each tile by hand like buttering a slice of toast and apply the tile in its place swiggling it around to mash the mastic all around. Then do the others the same way.
NOW, very important, and could be another cause of this problem. Do not grout for a good 24 hours after masticing the tile to the flooring. If you grout too soon, the tile mastic cannot dry out and the adhersion is compromised.
Vent the area well also.
PS, while your doing this, make sure any other tile that is loose or not snug is removed too.