PMM9BRO - The problem with using ready-mixed tub adhesives is that they depend upon moisture loss to cure. In other words, they need to dry out to be hard, whereas cement-based adhesivesa harden through a chemical reaction. When water-based adhesives are applied to paint, the moisture has nowhere to go apart from into the tile biscuit, into the wall, or through the joints to the air to cure. Moisture can't penetrate paint easily. It seems that the adhesive you are using is not drying out because there is insufficient water loss for it to cure. I think the size of your tiles are probably pushing the weight limits that a ready-mixed adhesive can carry also. I would seriously consider holding back the tiling, stripping the paint now that it is wet, making sure the plaster is solid and dry, priming with an proper primer (not PVA!), and tile using a cement-based adhesive. It's only advise...