I know this is a very old thread, but I found it while trying to troubleshoot my foodsaver with the same problem. In case anyone else runs across this in the future, here's how I fixed mine:
There's a pressure switch attached to the circuit board that senses the pressure inside the bag, and once the appropriate vacuum is reached, it triggers the automatic sealing process.
Once I took the unit apart and got down to the switch (it's a flat, reddish colored drum attached to the vacuum hose), it became clear that one of the legs of the switch had worked its way free from the board. The solder joint just went bad, so now the switch was no longer connected and not doing anything. I resoldered both legs to the board, and now the foodsaver works just fine again.