Many assume that once you pay a visit to the dentist for a filling, that tooth is fixed, and any future cavities will form in other teeth. This is not correct. A cavity can form under a filling and these are the reasons:

- There is damage to the filling
- The filling is old and worn out
- The filling is too large or small and the fit is faulty

"These fillings are best evaluated by a professional, and your dentist can tell you if your fillings are in good shape, if they need repair, or if you have decay underneath."
Read a detailed explanation here: https://yourdentalhealthresource.com/can-cavities-form-under-dental-fillings/