We recently had underfloor heating installed in the bathroom (Wiring)?

My husband did the floor tiling using the correct adhesive, thickness etc, but some of the tiles keep lifting and the cement will not dry out evenly. After two attempts and lots of expense we are at a loss as to why this is happening. It occured to me that the person who installed the heating perhaps ought to have laid it with matting over the top of it.

The installer says not, but what else could be the cause of the cement not drying on some of the tiles. Some have dried out perfectly well and are fixed others are not.

The concrete floor has the correct screen and was not damp prior to laying the tiles. Does anyone have any ideas please? Before we both go loco.

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