Avoid pulp capping as much as possible and in this case the reason for pain will be due to infection or incomplete removal of coronal pulp.
Next Steps
IOPA or RVG should be taken to diagnose the current status of the tooth and should be followed by RCT which appears to be more likely treatment from what I infer.
Health Tips
Explain the patients about technique sensitive issues related to pulp capping if your opting it and educate them with the future recommended treatments such as RCT or even extraction in case of failure.