Those pumps are usually controlled by a float switch. The switch makes or breaks the circuit when the float is 30 degrees (usually) above or below the horizontal. The float is on a cable attached to the pump itself (although it can be separately mounted). You need to make sure that the cable is not too long (do that e.g. by using the clip on the pump casing) or that float doesn't lodge on some part of the sump - otherwise it will not go 30 degrees below the horizontal.
JD, October 2009
