The altitude adjustment on the bottom of most models controls at what amount of vacuum the sealing element comes on. If you gum up the altitude switch it will either pull the air out or seal but rarely both. I could never get mine to work again reliably.
The easiest fix is plug off the vacuum line and put the switch outside the sealer. Then you control how much air is pulled out and when it seals. Test with a multi-meter which wires control what. On mine it was originally set so that I would have to hold the switch then release when I wanted a seal but I switched two of the tree wires and now I hold the on button to vacuum and press the new seal button when I need the heat strip to come on.
Blake
March 2007