If you notice that your pump begins to draw in air when you connect the hose into the skimmer, possibly drawing in so much air that the pump loses its prime of water, it probably originates from an old, dry rotted hose with holes in it, or a cavitating pump drawing air in through the plumbing or valving.
To check the hose, hold one end tightly against your thigh while you make a tight seal with the other end around your mouth. Blow into the hose; you should feel very strong resistance. If you can blow easily, the hose has one or more holes or splits in it, and you may be able to hear the air being drawn through when it's hooked up for vacuuming.
When a vacuum hose is hooked into the skimmer and perhaps some valves are closed to increase suction, we are increasing the "vacuum pressure" in the line, creating a front pressure on the pump. This can cause the pump to draw air in places it normally wouldn't under lower pressure. This situation should be corrected by locating the air source and making appropriate repairs. (Repair info, see pumps)