Waxman,
This usually indicates that the Sensit sytem sees something that it shouldn't. It won't let the machine vend. Here are some things to check:
1. Push the mode key (yellow button) and check for errors on the display. You may see "Sensit Blocked" or Sensit Discont'd error. That may help pin-point the problem. There may be other errors also, so push the "0" (zero) button to erase each error until you see "Acct Data" on the display.
2. Check the narrow lenses on each side of the hopper to see if they are dirty or have a dust build up. Use a soft terry cloth towel to clean.
3. Look for anything that might be hanging below the kickers or from some other part of the hopper that the sensors can see.
4.. The deflector at the top, rear of the hopper sometimes gets bent forward and down so the Sensit system can see part of it. Pull it up and back a little so it is not seen by the sensors.
Hopefully this will get you going again.
Uncle Sam