Normally when the pump is running, you can look through the  clear trap lid and see water and debris swirling around in the trap body of the pump.  Sometimes however, it is evident that the water is not moving through the pump very well.  You might notice that the flow of water coming into the pump is only a fraction of its normal flow.

There are several possible explanations for this reduced flow.  It often indicates a clog of some type, either in the intake lines (skimmers or main drain), or in the impeller of the pump.  This might also indicate a broken diffuser within the pump itself.  It could also be caused by a filter that is in bad need of backwashing.