I don’t have one of these so I can’t speak from experience but I’ve looked at the performance curves and technical data for you and they are one of the low power pumps that are becoming more and more popular these days. This pump’s performance curve shows that it has a very impressive output and a quick search for prices shows that it gives good value for money. Although I don’t have any experience with this particular pump, I can tell you about low power pumps in general and how to get the best performance out of your one.
The advantage of all low power pumps (obviously) is that they consume relatively low amounts of energy. A point to bear in mind is that no water pump can achieve its rated maximum flow rate if there is any back pressure. High power pumps are less affected by back pressure and low power pumps are more affected by it. The pressure of the water leaving the outlet port of a pump is dictated by the design of the impeller and the shape of the volute (impeller chamber) that it spins in.
When designing a new pump the manufacturer chooses a compromise between the delivery water pressure and the flow rate against the electrical power that the pump will consume.
Higher output pressures will maintain the designed flow rate over a wider range of back pressures but this means that the pump motor has to be more powerful in order to turn the impeller.
In other words, pumps that have a greater output water pressure need a higher power motor than a pump that has the same flow rate but with a lower pressure output. The second type is typical of the low power range of pumps that hobbyists are being offered these days.
When manufacturers of any pump test the performance of any pump they do it under the most favourable circumstances possible as shown in the diagram. We can’t achieve such a simple plumbing system on a pond because the pump has to overcome the back pressure due to the pipe run to any equipment it has to feed plus that of the equipment itself and that of the pipework back to the pond. Obviously the simpler the pipe run, the less back pressure that will be generated. The principles are well known; larger bore pipes with shorter runs and the least possible number of bends give better flow rates. However, there is a hidden force that increases the back pressure and, therefore, a reduced flow and that is the head height.
Head height is measured from the water surface (not from the bottom of the pond or from the pump itself) to the outlet of the pipe leading away from the pump. The lower the head height, the lower will be the resistance to flow.
Hope that helps.
Hope I've not jinxed them 
are you running them dry?