These oil pumps are positive displacement
pumps, therefore if they are engaged and turning the oil must be going somewhere. Start by ensuring that the drive is properly engaged and the gear
wheels are in fact turning. With the oil pressure sensor removed you should be able to turn the engine
over with the decompression levers lifted until oil is seen at the sensor and likewise for the filter. Once this is established you know the pump is working. Then look for an open plug
in the oilways such as the crank shaft bearings, head gear
etc, it has to be going somewhere.