You dont say where the thinset failed. If it is sticking to plywood and not the tiles, my guess would be that you applied the tiles dry. When using thinset for tile, the back of the tile needs to be damp so that the thinset does not dry out before adhering. If it dries quickly, the tile will come loose as you have seen. I assume in this response, the adhesion failure is at the tile, not the floor.
Rather than use the mastic, I would clean the floor and return to using the thinset, but soak the tile before replacing. Let excess water drain off as you butter the floor with thinset, then place the tile and slightly twist while adding pressure to set the tile. It should stick.
Finally, on your next project, consider using a cement backer board under the tile. It provides a more stable surface and is ideal for thinset and mastic adhesion.
