Hi,
The answer to this question could be very vast,as the possibilities are many.
1. Problem could be in the filling of the roots properly. Or,
2. Any canal could have been missed. Or
3. The problem could be in the gum and bone (periodontium) because of a faulty crown.
As I said earlier, possibilities are many.
Next Steps
I would suggest to visit your Dentist and ask him about getting a CBCT done. This would help both the doctor and the patient to understand the issue better.
Health Tips
Do not delay the process.