Stagnant water smell after flushing

The powder room of my new house has a St. Thomas Creation low flow toilet with Quattro Flushing technology. This was installed in summer of 2006. Recently I have begun to notice a stagnant water smell emanating from this bathroom. There are other low-flow models installed in the house; e.g. Toto and these bathrooms do not seem to have the same problem. The smell is particularly powerful if the door to this bathroom is closed for a period of time. The smell seems worse after flushing. Any ideas?

The vanity sink and/or toilet may not be properly vented, and when you flush the toilet, it sucks water out of the trap at the vanity, opening up a direct open-air path to the sewer.

The weakest joint in the plumbing system is the wax ring... I would reset the toilet for starters and see if that does the trick... If the toilet is at all loose the chance of the wax ring leaking jumps to 100%