Remove the racks and inspect the main riser in the back of the unit. it is a thin plastic channel with two sets of two ports. Each of these 4 ports should have a flapper that acts as a check valve. They can easily become dislodged, so DO NOT POKE your finger into the ports.
If they are already missing, they have probably fallen into the riser. You will need to partially disassemble the riser to remove it and shake it around to get the flappers out. It will be obvious how to reinstall them, although I recommend getting new ones.
If you have any questions, email me at jbyrne@maine.rr.com
Jim Byrne
July 2007