Most systems require adjustment to the valving to increase flow in the line through which you are vacuuming. You may want to close all the valves except the one on the line you are vacuuming through. On some systems, closing too many suction valves will cause the pump to cavitate, which occurs when it is starved for water. If the pump begins to shudder and make interesting noises, open the valves until this ceases.
If your suction still sucks, check that the filter is clean and the pump basket has been cleaned. Before vacuuming debris into the pump basket, always make sure the basket is locked into place properly so that debris cannot bypass it and clog the impeller.