First, make sure all manways and hatches are closed tight, on both the vacuum unit and any collection tank or vacuum box that you are filling. There could be an open valve, a loose seal, a poorly fitted camlock gasket, cracked hose, etc. If a liquid ring-based unit, make sure that the pump is primed. The primer valve should be open while the water tank is filling, and then closed when the unit is running. As a rule of thumb, if the belt is turning between the motor and the pump, there should be vacuum at the unit.