Date: 201702
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Pressurization System
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Human Factors; Procedure; Aircraft
Primary problem: Aircraft

Narrative: Finished boarding; finished before push checklist. All door and warning lights were out. We pushed normally. Normal taxi. All checklists complete. Takeoff was normal with First Officer pilot flying. Climbing out of about 3;000 feet MSL; we both noticed a pressure anomaly in our ears and looked up at the digital pressure controller and pressure gauge. Cabin pressure was climbing with the aircraft. The aircraft was not pressurizing. We made sure the wing was clean and gear was up. First Officer engaged autopilot. Because the pressure controller and air panel are on the First Officer side; the controls were then positively transferred to me.I was now Pilot Flying. We checked the air panel configuration. It was correct. We checked outflow valve gauge. Valve was closed. We had 40 PSI on each bleed gauge. I asked for and received a level off at 5;000 feet. This stabilized the situation. First Officer searched the QRH for an appropriate checklist. There is none for this situation. We decided to try the alternate controller and also the manual controller. After we cycled through all three control modes; I asked for a climb to 6;000 feet to see if there was any change. There was not and the cabin climbed to 6;000 feet. We immediately agreed to return to [departure airport].First Officer did climb check and started running numbers for landing as I worked radios and flew. We decided on a longer final to buy time to accomplish all checklists and assure we were under max landing weight. Among all the other things the First Officer was doing; he sent ACARS message and called Operations. All checklists were completed; the controls were transferred back to First Officer and we made a slower than normal descent to an uneventful visual approach and landing.We never sensed that there was any kind of damage to the aircraft. Normal taxi to gate with waiting Mechanics. They told us that the aft cargo door was not closed properly. Two latches engaged (these turned out door light in cockpit) and two latches disengaged. There was a large gap at bottom of door. No emergency was declared; no passengers were harmed; and no masks dropped in cabin; no crew harmed.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported being unable to pressurize the aircraft and returned to the departure airport where the cargo door was found to not be closed properly.