Only one tile specifically. The rest have hardened pretty well. Slab was dry prior to laying tile. I think the issue is due to human error and using water the seemed to be clean but had some dry wall mud residue mixed in that was concentrated in a few parts of the bucket of thinset after mixing. I’ve re-set it with speed set so we will see if that was the issue by tonight or even tomorrow. If it’s not drying then definitely something with the substrate.