I have one specific spot in my kitchen where the tile has sunk a little. It's happened right on the grout line, which is actually how I noticed it. I saw the grout cracking and popping out, and when I took a closer look realized the floor was low in that spot. I honestly can't say if the floor was low there when I first moved in. Just to make sure I wasn't seeing things, I brought out my large level and sure enough, that spot is visibly lower.

I have a basement so I measured out where this spot was and went down to the basement to check. There were no problems that I could see. Beams looked straight, plywood wasn't cracked or warped, and there was no visible water damage. Not that I could see *how* water could get there. It's well away from the sink and any pipes. I'm also the first owner, so I know there hasn't been any flooding. I've checked around the house, and found one other spot in office that was low, though it was barely noticeable.

What could be causing this? My current theory is the subsurface prep wasn't done correctly, maybe they didn't apply the thinset in these spots, or left too big of a gap in the cement backerboard. I know the boards are there because I checked while the house was being built. Really don't want to go ripping up tiles in the middle of the kitchen when I don't know where to even start for repair.