If the pumps behave erratically, it is very often caused by improper priming. If air is still inside, which is on the one hand compressible and on the other hand uncontrollable to know when such an air bubble starts to come out, the flow rate can vary a lot.
NOTE: For priming run the pump at max settings. The beginning of the priming may be difficult as plastic materials are in general hydrophobic, so that air bubbles may remain inside.
If the liquid is not degassed and remains inside the pump tiny gas bubbles will appear on the interior surfaces after some time. It is the same effect as in a glass of tap water. These gas bubbles are compressible and eat up a bit of actuator stroke and compression that lead to decreasing pump performance.