What glue should I use for diamond art?

While you work, none: the canvas already has its own glue. If an area has lost its tack, clean it gently with sticky tape, then if needed brush a thin layer of clear craft glue on that spot only.

For a single drill that has popped off, a tiny dot of clear glue on the tip of a cocktail stick is enough. Once the picture is finished, what you want is a sealer (see the sealer question).