slate in the shower

We tiled a shower with slate tiles 12x12 using half in durock behind them and mapei thinset ultralight 25 and this morning I had a couple of tiles fall off. Thinset had deffintely cured to the walls but no adhesion to the tile only a few specific ones though. Could this just have been a fluke? They were deffinatley pressed into the thinset enough and u couldn't pry them off at the time of instalation. So that was quite a surprise. Is it the thinset or the tile or had it just set 2 or 3 mins to long?

Are the tiles ungauged? Different thicknesses?

You are supposed to back butter slate too. This is prolly what the problem was.

If the thinset sits on the wall too long after being applied, the tiles will not engage the thinset properly.

Slate tiles are dusty and should be first sponged-off with a wet sponge then backbuttered with thinset (flat side of trowel) before applying to the wall surface with the thinset.

Sounds like "installer error".

Ultralight thinset has a very short pot life, you need to mix it thin on your first mix otherwise you will have probs like you described mix your mud thinner also sponge your cement board down so it doesnt cure so quick

Make sure you seal the slate very well when you are done.