Things to check for:
-How many exhaust vents do you have on the roof?
-Do you have intake ventilation along the eaves to introduce air into the attic?
-If so, can you post a pic of that?
-Is the area around the eaves stuffed with insulation, thus restricting the air flow?
-Does your bathroom exhaust ventilate out through the roof deck or does it terminate into the attic?
-Have you noticed ice formed on the underside of the roof deck during freezing temps? Can you post pics of that?
-Do you have standing water in a crawl space under the house?
The reason you want to close off the other vents, if you have any, is it will short cycle the attic exhaust. Meaning, when the fan comes on, it will draw air in through the exhaust vents and not pull warm, moist air out of the attic from the soffit area.
If you have a power vent installed, make sure it has a humidistat control and a temp control.