Sand-blasting cabinets?

Glenn, from your description it almost sounds like there's a hole in the uptake tube (the [presumably metal] one that gets buried in media) that's large enough to lose the vacuum suction. If the gun is spraying air, it doesn't sound like it's the problem. If your lines are not metal, you may be experiencing a tube collapse situation or the lines are plugged up.

Try pulling the trigger while holding your hand over the nozzle of the gun until you see a little cloud of media appear in the cabinet. This will dislodge any clogs in the lines.