When your machine is connected to your tank, does it actually work? I mean does it actually spray water like normal?
If not, i had this same problem when i was helping set up a mates washer. hooked everything up, started it, pulled the trigger and no water came out. turned everything off, and undid one of the big nuts on the pump head that cover the valves, bit of air came out, tightened it all back up, started it again and it all worked.
As someone said, air lock. If your pump head doesnt have a bleed valve, ive found this is the best way to get rid of the air.
Worth a shot. But this wont work if the machine is spraying like normal when connected to the tank.