Unfortunately it is impossible to say whey this keeps on happening. Ther could be many reasons i.e the foundation is not strong enough, there could some form of interference when biting, etc.
I would vist the dentist again and express your concerns and see what are your options.
Dr Kalpesh Bohara
The Dental Suite
36 Frederick Street, Loughborough Leicester, LE11 3BJ
Web : www.dental-suite.co.uk Tel: 01509 231144