Floor tiles cracked in places

[springmachine]

New tiles floor laid in upstairs bathroom this time last year.

Tiler put down 12mm ply first and tiled onto that.
Underneath was pine floorboards.

The tiles are Laura Ashley porcelain 30cm square tiles and there is one run that have cracked in a straight line across the middle of the tile

This could have happened a while ago as it's only hairline cracks and I don't crawl around on the floor.

Any idea why this could have happened and is there an easy fix?

Is there a joint in the ply under the crack? Has the ply been screwed down at regular intervals?

Ours are doing the same thing! It's so Annoying and is getting worse. We have under floor heating in I have no idea of how to take them up without damaging the heating. Any ideas much appreciated!