I am tiling a bathroom for the first time. It is an upstairs bathroom so I pulled out the vinyl and luan. The subfloor met the standards that I found else where on this site. I laid hardibacker board with total flex thinset and then screwed it all down. I then taped and used thinset on the joints. I started laying my tiles yesterday and everything was going fine until I reached an uneven spot. It is at the sink area on one of the joints. I have sanded that area down as much as possible, but the tile is still able to make a rocking motion when I lay it down without any thinset. I already cracked one with thinset on it when I tapped the block with my rubber mallet. So here is my question, how do I make sure the tile does not crack? Do I build up the thinset in the low areas? Wouldn't that make the tiles higher than the surrounding tiles?
Edit- this is the case for 1 tile only