The DD601 is getting fairly old now and it is possible that the air pump or the seal is not as good as it used to be. Without the proper test pressure gauge it is near impossible to check if it is working ok. It could also be a problem with the lid actuators which pull the lid down on to the tub. Another possibility which you can check is that the wiring & drain hose at the rear of the drawer is moving clear when the drawer is closed. You mentioned that the drawer does not close the same as the other one. Remove the drawer from the cabinet & inspect the harness & linkage for problems. You will probably have to get someone in to check the seal is ok.
Lincoln Appliance NZ, July 2009