it is hard to say. but there are several possibilities. the veneers may be made out of porcelain or composite resin. they can either de-bond where the chemical adhesion breaks down or they can break off. this can be caused by a traumatic event grinding or clenching. my best advice is to discuss it with the dentist who placed them. you can also call your local dental society to ask for help with a referral.
the veneers on my two front teeth keep coming off. why is this happening?