After many years (14?), ours were just starting to just come up around the shower. We kept spares but the adhesive on the spares backs had failed also. I'm guessing the adhesive has a life, in use or not.
lucky, Lowe's still has the same pattern if needed. I just used some Life Caulk and glued the new ones down. There was just some color fade in the old ones, the new / old spares look great.
What we learned years ago; Supper clean surface to stick to. Supper flat. If your going over plain wood deck, seal it and lightly sand supper flat and smooth. I removed a lift ring on a hatch and the tile caved in on the hole within a week. Some filler and a new (then) tile and all was great. Supper sharp razor knife with plenty of fresh blades. Use a steel rule to cut against on top of scrap ply wood. You can not hand cut straight lines. Not a wimpy cutter, a big handled one, you need to make the cut in one pass. Don't install anything until the puzzle is complete. Any rounds or curves make a template from cardboard. Rough cut out the tile wide. Use a 1 or 2 inch dowel with sand paper on it and shape the curve lil at a time till it fits.
Have fun.
