I have had a few boats with Niad stabilizers and I believe if you have fluid and the pump is turning then the problem is either in the controls or a bypass valve. A small leak is not going to allow enough air to airlock the pump. And unless it is at the top of the system it would fill when shut down.
If you have replaced the filter and still have the old filter, cut it apart and look inside to see what is in it. Dirt is the main cause of hydraulic malfunction
Is there any noise
? Hydraulics are noisy and a cavitating pump or a working bypass valve should be squealing. If no pressure and no noise
the fluid is likely flowing freely through the bypass valve with no pressure or restrictions.
Again, check the controls and gyro, if no signal to the valves, it simply free flows.