damp concrete floor (1st floor)

Hello all, I am wondering if anyone can help me with this problem.

I recently had a cork floor laid over my concrete bathroom floor. The floor was dry when the tiles were laid but within a week the tiles were coming up and when we looked underneath parts of the floor were damp (which I think was causing the tiles to swell/ unstick maybe). My problem is, where is the damp coming from? It's a 1960's concrete block, The bathroom is on the 1st floor and their is nothing underneath it (ie the bathroom juts out above the front door). Their is no damp coming through the walls. I've been told it maybe condensation/ spillage from the shower but I have just had an extrator fan fitted, plus I always leave the window open when I shower and theres never water on the floor as theres a screen, mat etc, plus I'm just tidy that way (!) The cork tiles were also sealed with 4 coats of polyurethane varnish, with me paying special attention to filling all the gaps.

Any thoughts would be much appreciated, thank you

Before you had to cork installed, what type of floorcovering did you have?

hi, thanks for getting back to me
before there were ceramic floor tiles, the builder took these up, then put the cork tiles straight onto the concrete foor with adhesive

The moisture has to be coming from when the bathroom is in use if there is nothing obvious penetrating and you are on the 1st floor.

Urethane is the correct sealant for kitchens/bathrooms, so ok there.

How have you sealed the edges around skirting, w/c, pedestal etc ? and is your shower/bath well sealed ?

Just to note, cork flooring needs to acclimatise to room conditions and any previous adhesive needs to be completely removed or a screed laid over the top.