Today, I found a pair of tiles had lifted to form a ridge where they join. OK, only a few mm but to do that, they had clearly lost all adhesion. I have scraped the grout from their edges and removed some of the tile cement beneath, so I could tape them back down for the moment, as I'm severely short on time this weekend. A brief investigation shows quite a few tiles in this area are loose, so maybe a square metre or so will have to be taken up, cleaned and re-stuck.
These tiles are not new but are certainly not as old as the house and I would estimate they are a decade or two old. They were undoubtedly laid 'professionally' (as in someone took cash to do it, not necessarily someone who knew anything) on a floor described to me by a builder as polished concrete, which seems quite robust (but who can tell?). The adhesive looks more like mortar to me than anything I've used.
So, questions: Why would this happen suddenly? It's not damp and there haven't been any serious temperature changes, neither is it a part of the floor that gets significant traffic. In other words, should I look for a cause or just put it down as one of those things?
How much do I need to clean the tiles up before re-laying? Getting the grout off is easy, as is removing the adhesive from the floor. Although any big chunks of stuff can probably be flipped of their back sides, I'm a bit reluctant to start bashing the tiles too hard to get all the adhesive off them as they would be imposible to match, were I to break one, and they're an odd size (probably 8 inches-ish square) to even source a non-matching replacement, without cutting a larger one down.
Any thoughts gratefully accepted.