Make sure battery is fresh. If there is any doubt, try a new one (where you have to remove the sticky flap off the battery surface, let new battery stand for 1 full minute before inserting into aid). Make sure volume control is on (volume dial should be set above the number 1).
Check the tubing for any signs of moisture, debris, or ear wax blockages.
Check the battery contacts to be sure they are not corroded.
Reposition the hearing aid tubing/dome in your ear canal.