Reasons for pain after RCT can be many. It also depends on when the treatment was done, what was the infection level before treatment and how was the healing before cap/crown placement. Under obturation may not be an issue always and even if it's the cause can always be retreated. As for the crown dislodgement it can be cemented back if it's acceptable or evaluation of the reasons for dislodgement and correction is required.

If you have the xrays which can tell that the RCT has not been done properly, you can talk to the dentist about the same. There are many reasons of under obturation during RCT which include negligence from the doctor and even sometimes the anatomy of the root canals. Even for the dislodgement of the Crowns, the reasons can be improper fabrication of the Crown or improper Cementation of the Crown or even like accumulation leading to acid formation. If you are not satisfied with your doctor you always have the option of going for a second consultation. You can search for an endodontist in your vicinity using Practo and contact him for the rectification of your problem. Regards.