I thought I answered this but i don't see the response.
The job the the check valve is to check pressure differential between the pressure that leaves the tank and the pressure that stabilizes. The valves have to be completely off for this to happen. If you have reset correctly and still have problems there may be a small leak.
Many homeowners mistakenly believe this is a regulator problem when it is not but changing the regulator will reset the check valve in the tank so it appears to fix the problem.
Usually this happens where the valve seals to the manifold but and gas fitting can have a problem. Spray the fittings and the valves with soapy water to see if you get bubbles.
Other common problems that appear to be low gas are faulty tanks, mis-adjusted valves and insects.
The empty space behind the orifice can be a welcome area to build a home for a tiny insect. Remove the orifice and blow out the orifice and the valve.
The control valve should have a tiny set-screw in the valve stem. Use a small flat-gead to adjust the gas while the burner is "on" and make a note of each adjustment so you can adjust back as the burner will adjust slowly. The air shutter can also be adjusted while the burner is on and this carburetor has to allow an air mixture or the gas will not burn, or will burn low.
If you have been exchanging tanks many exchange tanks have faulty valves. However, like the regulator if this is "bad" they will usually not work at all.
Finally, If the tank is overfilled the displacement caused by the OPD float can trigger the check valve over and over regardless of how many times you reset the pressure or how slow you open the valve. This will seem like low pressure because the initial reading of the check valve is too high for the pressurized stabilization to match it and the check valve reads the difference as a leak.
There is a screw on the side of the valve opposite the pressure release (indented triangle) that will expel gas but you're better taking this to the fill-station so you're not standing in a cloud of flammable vaporized propane.