How thick is the ply and is it adequately fixed down so there is absolutely no flexing? The timber floor the ply is fixed to should be checked and improved before the ply is fixed and the ply screwed (not nailed) down at a maximum of 100mm centres. If the substrate (i.e. the floor and the ply) is insufficiently stiff, flexing will loosen the tiles. A grey cement based floor tile adhesive must be used – you can buy some that are described as “flexible” or use an additive on standard adhesives, that will allow for a small amount of movement. Special primers can also be used on the ply to maximise adhesion, but that doesn’t sound like a problem here. If the ply is flexing to any great extent, I’m afraid the tiles will come loose whatever adhesive you use.
