Pour wallpaper adhesive into a glue-injecting syringe. Position the syringe needle at the middle of the air bubble on the wallpaper.
Push the needle into the air bubble. If the needle cannot penetrate the wallpaper, cut a tiny slit with a utility knife into the middle of the air bubble. Stick the needle into the slit. Inject the glue slowly, using only enough to fill the bubble.
Remove the syringe. Push the bubble flat with a wallpaper seam roller, gliding the roller up and down.
Wipe the affected area with a damp cloth to remove any glue residue. Dry with a clean cloth.