Do not adhere your tiles to plywood directly because it will.
Cracked floor tile solution.
If you have ceramic glazed tile and cracks here is an easy fix.
Work loose as much grout and adhesive from the edges and underside of the tile as you can as well as any residue left on the flooring.
Replace the grout joint around the edges of the tile.
As necessary reinforce the subfloor with plywood sheeting.
This will prevent formation of cracks in the concrete which lead to reflective cracking of the tile.
For a wooden subfloor make sure the wood resists sagging and bending.
Pour a little rubbing alcohol onto 1 or 2 sheets of paper towel.
Another solution is to install a crack isolation membrane on top of your existing concrete subfloor.
Re apply adhesive to the back of the tile and set it back in place.
Find a nail polish or a combination of polishes and mix to match your tile.
Cleaning the cracked tile will remove any dust and grime and ensure that the epoxy sticks to the surface of the tile itself.
If not the tile which is extremely brittle could crack when walked on.