First, given the number of sewer smell questions, I want to point out that I have searched a ton to try and find an answer to this question and I didn't come up with anything.
I have an issue on my top 3rd floor bathroom where I get a sewer smell in the shower but ONLY when the shower is draining and then only after 5-10 minutes. I.e. the smell comes only when the shower is in use and then only after a period of time. I don't get a smell from the sink or the toilet in the same room. The problem is intermittent.
I cleaned the drainage pipe to no avail and I checked the drainage of the trap when being used and it looks to be draining fine (i.e. water is filled the entire time and thus there looks to be a seal with the trap from gases below).
Problem occurs when the fan is on and when it is off. Only happens in this bathroom. I have noticed that the toilet leaks a bit at the seal (though no smell from there) and I am operating on the theory that the two problems are separate but thought I should throw that in.