"Missing cap" indicates that a lead was received outside of the configured cap times or days. This means the system did not apply the usual lead capping rules at the time of submission.
If the lead is currently in a quarantined status (or another status, depending on how your cap is configured), you can try reprocessing the lead. This may move it into a received or accepted status.
Note: An accepted status typically only applies if your campaign has a forwarder attached.