Tiling on a wooden floor

[stegga]

I have just laid a ceramic tile floor for the second time. This is a wooden floor. The first time it cracked up due to the board i tiled on being too thin causing movement, (first timer). I then researched how to do i properly and followed the instructions to the letter, it has cracked up in the same place!!!
This time, i had laid it on 12mm ply, almost overly screwed down, used flexible adhesive, there was no movement in the floor. Three days later, movement in three tiles.
Can anybody help as i seem to be throwing good money after bad here.
Cheers

[stegga]

The ply i used was 18mm not 12mm as i stated in the question
Cheers