Multiple issues. Spot set is the completely wrong methodology. Backbuttering to a minimum of 85% coverage is required, as well as troweling the wall. No blobs, leaving voids. It also likely skinned over because of too much time and wasn’t fresh enough to offer a bond. “HD thinset” doesn’t say much about brand and proper adhesive qualities.

Plus there is no grout to lock the installation together, as is required. A total $500 craigslist hack job. And he doesn’t have the skills to do it right in the first place. Move on and hire an actual Pro. The way the tile is coming off, you might even be able to salvage it. But, the rest of the demo is only going to add to the eventual real world costs for doing a backsplash with a Pro.

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

Thank you for your feedback. Attached is photo of the thinset. We were trying to decide between Clear/diamond grout or similar grout color to the glass tile, hence the delay in the grout install. Should we try to take all tiles off. Or about how long should we wait to see if anymore fall off? Thank you again Sophie.

Here is a link to google, how that backsplash should have been installed. There are some u tube videos too. I watched one video, where they used grout, between the countertop and the backsplash, and grout should not be used there. Caulking shud be done between backsplash and countertop. My backsplash is also caulked in all the corners, rather than grouted. Once you watch these videos and read these articles, you will see that your thinset was put on improperly, and thats why your tiles are falling off. Please read carefully. https://www.google.com/search?source=hp&ei=uI4VW7HdLszKjwTtrqSoAw&q=how+to+install+glass+subway+tile+backsplash&oq=How+to+install+glass+subway+tile+&gs_l=psy-ab.1.0.0l3j0i22i30k1l3.3755.12143.0.14244.41.29.3.9.9.0.173.3348.9j20.29.0....0...1c.1.64.psy-ab..0.41.3556...0i131k1j33i22i29i30k1.0.70wb9L5GlO8