If you have clear lines look around the tank select valve and see if you can see bubbles on the tap side, the valve may be letting air in................. Suction can draw air into system where no water will leak.
Also check line filter and switching tanks, there could be a bit of drill swirl blocking the tank outlet, it will cause splutter and low flow, can pull a line off and blow down the line into tank to push swirl from inlet, then hook back up and try pump, if flows good then you can just wait till next time or remove tank and shake the swirl out of the inlet hole is the best way to clear it from tank, it can be there for years...
Another cause can be a blocked breather, no air getting into tank to replace water removed will slow the flow if you have sealed inlet.