If you or the hearing people in your life complain about feedback from your aids, here's a list of causes, diagnosis, and what you can do about it. This is a defect, but is doesn't occur very often. This should be visible upon close inspection.
Solution is to get a new mold.
Coating the earmold in mineral oil or petroleum jelly may provide a better seal and may be a reasonable short-term solution. Venting in the earmold is another big cause of feedback. The solution is to partially or totally plug the vent, if this does not interfere with the user's amplification needs. Wax in the ear is a common cause of feedback. This is not very common, but can happen.