BC, Bekka,

If the water is dropping in your pump basket, like bekka's, you are sucking air into the system when you hook up that vacuum.

Look in the skiimer where you have attached the hose and see if that connection is remaining submerged (not letting any air in). If that's not it, check your hose connections. If that's not it, your search needs to be from the head of the vacuum to the connection of your hose into the skimmer. The air is getting in somewhere between those two points. Post back if you find it and let others know.