Bump in floor

[FredB]

We had some vinyl flooring laid in our shower room about 3 years ago. Before they did it, they laid some levelling screed.
It has been fine, but just recently a bump has appeared under the vinyl. As the vinyl is sealed at the edges, we could not imagine what might have got under the vinyl to cause the bump. The bump is about 4 to 6 inches across and probably about half inch high in the middle.
We have just lifted the vinyl and are surprised to find that the screed is actually raised as a bump, not cracked. And we find that an area around the bump sounds very hollow when tapped.
We now think that perhaps the screed was laid on thermoplastic tiles, and that perhaps something has lifted. Does this sound likely ??
Before we start to dig the whole thing up, has anyone seen or heard of anything similar, and if so, any tips to remedy ??

Thanks - FredB

 

[mattysupra]

Sounds like water based compound over damp floor or thermo plastic's etc. Also can be caused by no primer or wrong primer. And you will have to chip it all back up until you find a hard spot. Just get the firm back who laid it.