Blowing out water lines

I purchased a 2022 T@B 320 Boondock and the dealer kindly agreed to drain the water system and blow out the lines. Yesterday I decided to do it myself and make sure there was no water in the system. I followed the NuCamp instructions and was able to pressurize the system (40 PSI) and blew out a small amount of water from the faucets and low point drains. I decided to try it again today (for practice) and followed the instructions again. However, I couldn't get the system to pressurize. I tried three or four times, carefully following the instructions. The yellow flip valve for draining the hot water tank was closed, all faucets were closed, and the low point valves were closed. When I stop adding air, the pressure quickly drops and sounds like it may be coming from a partially pressurized tank. If I open a faucet or one of the low point valves with the air compressor hooked up, no air comes out. The instructions said nothing about the hot water tank bypass valve, but I tried it in both bypass and normal mode. Nothing changed. I'm very puzzled why it worked yesterday but not today. Any thoughts would be most appreciated.