First make sure that all the traps are full of water. If they are, disassemble the tailpiece under the sink before the trap, and remove hair (if any). If smell persists, there is a crack or hole in your vent piping.
When the water in the trap portion of your drain evaporates, this removes the natural water seal and opens your home to disease carrying bugs and sewage gas smell. H2GOFlo and the theInterceptor-Plus replenishes this water periodically, creating the required barrier, that blocks off the back flow from the sewer system from having direct access into your home.