QUESTION OF THE WEEK: Sewage Odors

I am having ongoing battles with sewer odors in several buildings. We have attacked this issue from several sides without a cost effective total solution. I have treated drains, traps, and grease traps. I have checked for air balance to ensure no negative air pressures. I have replaced grease traps. I have installed air emittance devices on vents. In cool weather the odors still seem to invade two of the buildings. What am I missing? I know there may not be a silver bullet for these, but any suggestion are welcome.