This is typical of jobs we do. Grout haze is the sign of doing it the wrong way round. I think most of the Tilers on here will agree, Seal First, Fix the Tiles, Grout and seal again. If everyone did it this way, I would be out of work. Thankfully, there are builders and plumbers out there who know better and I keep busy.
I tiled for many a year and to be honest, I would not expect anyone to get paid for a job like that, but like someone has already pointed out, if the slate is badly cut/calibrated, then the best of tilers would struggle with the work. We used to re-cut badly calibrated tiles to suit, but you need patience and time to do it.
