In your construction you probably have vent stacks for each of the three areas. If, as you say, vertical units off one vent stack all have the same problem then it is most likely the case that their is some larger construction problem.
If your fixtures are not trapped then you would probably have this problem everywhere. Yet I don't understand why you added traps to your unit? My diagnosis would go like this:
1. Are the fixtures and drains in the problem units properly trapped?
2. Are the fixtures and drains in the OK units properly trapped?
3. If they aren't trapped, supply traps.
4. If they are trapped or the problem persists then you are leaking sewer gas, not through your fixtures, but through your piping. There would likely be a chase in which this vent stack is located. By opening the chase you could verify if there was a stronger aroma in the chase than elsewhere.
Beyond this, the problem may be expensive to fix. You may consider having some pro's over there do a pressure or smoke test on the vent stack. I'd first make sure you have traps.