The Old thinset residue will not hurt a thing as long as its not built up to where the tile may set too high. Moisten everything before you reapply the thinset.

shw001, why did the tiles come loose?

Reason I ask, if they loosened by themselves it could be an underlying problem that will happen again. Like, water seeping where it should not.

Thanks Don, that's reassuring. Will give it a try

Measure Twice, that's a great question! and a mystery I have been wondering about. The installation is 2 years old. About a year ago about 4-5 inches of grout broke out in another location on the same shower floor (no loose tiles), I re-grouted with leftover grout and it still looks great.

This new section started with grout cracking in a line about 18 inches long, then the crack turns 90 degrees and goes for a few more inches. only 2 tiles are actually loose to the touch. I suspect bad mixture of thinset or grout in that area. Or maybe some hollow spots in the thinset under the tiles. The cracks have not grown in a couple of months and we use the shower every day.

I am expecting the worst that when I start scraping at the grout, about a dozen tiles will come out. Then I will use my ocillating tool to scrape out old thinset, clean up well, and re-set.

Thanks again everyone.