Ceramic Pencil trim tile fell off

[doublebonus]

Installed 5/8 ×1/2 x12 pencil trim tile with Versabond yesterday. Tiling is over a alcove bathtub. The trim is to finish the edge of tile that extends a few inches outside of bathtub. I was doing a light cleaning of excess thinset with a sponge and 2 of the tiles fell off. I'm worried that at some point more will fall off. Can I use some sort of tile adhesive or construction adhesive that hold them on the wall? They are outside the confines of the shower/ tub.