These are the 6 major reasons why your crown keeps coming off:
1-The number one reason for dental crown keep coming off is lack of retention, and tooth structure to hold the crown without the cement.
your tooth might have been over prepared period, and if there is not much tooth structure left the crown keeps popping off, there are possible ways to correct the underlying tooth structure.
2- There could be decay, eating away the tooth structure, and yes a tooth that has been crowned can get cavity, coming from the root, or possibly all the decays have not been cleaned up.
3-Your crown fit is not perfect and it is not fitting your tooth properly.
4-Your bite on the crown is not adjusted, you are putting extra excessive forces on the crown while chewing, and the occlusion on the crown is wrong.
5-your crown has been cemented vs bonded to the tooth structure, or possibly you have a post and root canal which was not fabricated properly.
6- Your crown length and width can not be supported by the root and prepared tooth! meaning it is bulkier than your original tooth, if your crown is bulky and the tooth is not prepared properly by the dentist the crown will exert unnecessary pressure on the tooth causing it either to come off, get loose or possibly damage the nerve.