I have wetroom installed in a loft conversion. I know that the plumber put in a wetroom deck and waste system from Wickes because he submitted the invoice to me. Later I had to fire him because his tiling and carpentry were so bad. The tradesman I got in to finish the job was generally unimpressed with the standard of the job done so far and re-did a lot of the plumbing.
My problem is that after sustained use of the wetroom you can see water seeping UP between the tiles when you walk across the room. I am worried that water is seeping between the tiles and being trapped by the deck. I suspect that there is too much "give" in the floor which means the grout between the tiles cracks.
How big a problem is this? I am selling my home and don't want to leave a problem for the new owner.

can you explain your bathroom a bit more.or even some pics might help do you remember what method of fixing was done (substrate waterproofing adh ect)
the more we know the better we can advice

Not to put it mildly - but you have a BIG problem.
Cracking joints/movement/water coming up - all mean the tiles will have to come up.
The only positive I can read is you didn't mention water going down to the ceiling below from the loft conversion.
Another issue you will now have is which of your wetroom 'specialists' is at fault.

How long has the 'wetroom' been in?

Welcome. You will have to remove all the floor tiles.The water would have done damage all over.Until you find the root of the problem,you cant go forward.

Same as Del and Timeless. Seen it before, leakage through tiles or flange around waste. If you cant see where its getting in th easiest way is to take it up and start again. Check the vertical and horizontal joints, they tend to go first. Sounds like the tanking works at least!

As above, pics needed to be more helpful.

check to make sure other pipe work is not leaking as if thats the case it will go the lowset point in floor which is the trap in shower floor , which will cause cracking grout and water coming through joints , if they are fine then its more likely to be the seal around trap under the tiles or seal where wall tiles meet base of tray