Hard to say for sure without knowing about your pool. It could be that you aren't getting the hose completely full of water before connecting it, or it could be that the pump just draws too much water through the skimmer for the vacuum hose to supply it. If you have a main drain or another skimmer, set the valves to feed some water through those. If not, there are vacuum plates that can be set to allow some water to flow directly into the skimmer.