There could be another reason for the observation that the main drain is not doing anything. It could be a function of friction losses in the piping. If there are two skimmers in addition to the main drain the pump is going to pull from the point of least resistance, so all the water may be coming from the skimmers. If you partially close the valves between the skimmers and the pump it will force the pump to draw more water from the main drain. As a test you might want to close off the skimmers totally, when you do that the pump should pull exclusively from the main drain.