There's an RV fitting/plug you can get to blow out your city water fill lines with an air compressor that looks like a tire valve, with a garden hose fitting on the other end. Do you have one of them? You mentioned you "blow out the lines". If so, like enblethen suggested, screw it into the city water fill, and give it a shot of compressed air. If you do use one and it was used to "blow out the lines" you may have introduced a small bit of debris that's holding the spring ball check valve open slightly. Or use a garden hose and give it a push/blast that way.
Mike. Comments are anecdotal or personal opinions, and worth what you paid for them.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 Wrangler JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)