Ceramic tile is known to do stuff like this (tenting) because it is slightly porous. Porcelain tile has LESS chance of tenting because its porosity is extremely low - almost impervious to moisture. It is much lower than ceramic. And yes, it can take as much as 10 years for a "bad" install to show itself in wet conditions. It can take that long to get the tile so wet that it's expansion is so great it breaks the bond of the adhesive. And yes...cut back adhesive is a poor substrate to bond to (for the Original Poster). Both of these photos (OP and andreadell79) are complete redos. Once a section starts to tent, you have to assume the WHOLE THING is going to go to he!! in a hand basket. Sorry but these are complete redos.

This is just a guess, but I am wondering if the edges of the handle are hitting the wall's trim kit plate when you push it in. (i.e maybe it needs more room to be turned off). If it is hitting, then maybe you just need a bit more room by tightening the trim kit screws a few more turns (they're visible next to the handle on the trim kit plate). Don't overtighten them, just what you need, since it will pull the diverter (behind the wall) further out. Alternatively, if it is a Moen, call their customer service for advice. Don't tell them you "just moved in" or anything, but say that you "have this, and it doesn't work".... Please let us know how it works out. Good luck.

I've installed trim for over ten years---wood and plastic---and never had that problem. I smell sumpin fishy. In the several houses I have worked with age like that, there were problems finding studs/top or bottom plates sometimes. Add to that installing over plaster/lath walls and you have a real situation. If the trim were shop made shortly before installation and not allowed to acclimatize to the house before installation---and only has finish on the front side, there is a chance the contractor could be right. BUT, that kind of trim/finish should never have been done, IMHO. I usually used 2&1/2" finish nails in 16 guage size. And often shot the nails at an angle.

Thanks for your replies. Sabrinacarr- the way I applied was simply to slather on thinset in a small area and apply the tiles to that area, then move to another area and do the same. Nicethyme- I used different tiles on different finials and one of the tiles I used was one that I had used on a sealed ceramic pot and they stuck just fine. So I don't think it was an oily problem this time, although I'm going to remember that, because I don't always clean my tiles well before using them! Just take the plates from my garage stash, wipe the dust and start nipping! I'm so bummed about these finials because they were really cute, but they took a while because the pieces were pretty tiny. I was going to put them on top of stakes and put them in amongst the flowers in the garden. Since they're for outdoors, that's why I was using thinset vs weldbond. So, I think I'll clear one off, seal it and try again. But just one at a time and see how that works. Thanks everyone!