I think he has pressure in the Tank? wouldn't think the fault to be the check valve then, What about pulling the regulator apart and checking the diaphragm, Also blow through the fuel Ports within the carb and Diaphragm with compressed air with needles out (after you Log where they are at)and (with the diaphragm out) and make sure all loose parts are out!! I have seen this more than once... ? I agree with Chris, send some pic's as well.
good luck,
Best Regards,
Kevin