If you have tried scrubbing and scrubbing, even with a scotchbrite type scrubby pad then you may need to resort to the big guns, a weak muriatic acid/water solution. This stuff is widely used for general masonry cleanup of residue mortars etc.. CAUTION!!! You need to mix this stuff correctly with water following the instructions. If you splash this stuff around it can badly injure you as it is ACID. Especially since you will be working over your head you need to have full protective clothing and eyeware etc. and protect adjacent materials etc.. I am not taking responsibility for you using it safely and correctly, that is up to you. I just think that this stuff will work if you have stubborn thinset mortar on your tiles.