As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SFP INFLIGHT T/1608/19 FLIGHT RETURNED TO STATION. UNABLE TO PRESSURIZE CABIN. LANDED WITHOUT INCIDENT. FOUND FORWARD CABIN DOOR DID NOT SEAL. DOOR SILL AND DOOR CHECKED NORMAL. PRESSURIZED AIRCRAFT TO 5 PSI AND NO LEAKS NOTED ON ANY PART OF THE L1 DOOR. CLEANED SEAL AND SILL. SYSTEM GROUND CHECKED NORMAL OPERATION.