What was found
(CAN) AIRCRAFT DEPARTED AND RETURNED TO BLOCKS DUE TO NOT BEING ABLE TO PRESSURIZE. FLIGHT CREW REPORTED THAT THEY COULD HEAR AIR RUSHING FROM THE CABIN DOOR. MAINTENANCE INVESTIGATED AND FOUND THAT ICE BUILD UP IN THE SUPPLY LINE TO THE CABIN DOOR SEAL WAS CAUSING IT NOT TO INFLATE. ICE WAS FLUSHED OUT AND THE DOOR SEAL WAS TESTED, AND CHECKED SERVICABLE. ACFT WAS RETURNED TO SERVICE.