Smelly Saniflow?

I have an inside bathroom where the shower, w/c and sink all go thru a saniflow. The light is linked to an extractor fan and there is a persistent smell that comes from the room when it is used. We have had a vent fitted to the outlet pipe to the sink but the smell appears to occur when the water is drained from the sink rather than after using the loo or shower.
Anyone got any thoughts on the cause not to mention the solution to the problem?