I would look at a couple things. 1. Inspect the gasket at the bottom of the furnace head. Make sure it is fully intact and there is no debris keeping it from making contact with the furnace base. 2. Inspect your vacuum hose and make sure it is not kinked, has any cracks/holes, and that it does not collapse (suck together) when the vacuum engages. 3. Make sure the vacuum pump is coming on and staying on. Inspect the power cords to make sure everything is in good shape and that the cord is not loose where it attaches to the furnace.
If it is not one of these things, then it could be a seal inside the furnace itself, and you would want to talk to equipment repair to see about getting those replaced (800-533-6825)