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How long do you run the fan for?

This is my question. I would guess it should take about 10 mins to exhaust the bathroom -- after a shower. (many variables there, just an approximation) If paper sticks to the grille, it sounds like it is working.

How big are the openings on the cap? If you are going a larger diameter cap, it will reduce the pressure significantly so it is possible the fan simply cannot push the air out past that pressure drop.

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I believe the cap diameter is 8" and the vent ducting is 4" diameter. Any way to test to confirm your suspicion?

I'll let it run while taking a shower and for 10-15 minutes afterwards. The moisture is just collecting on the ceiling excessively and completely (not partially) fogging over the mirrors. I've never lived anywhere where turning on the fan didn't have some impact, and I genuinely do not notice a difference from when there was no fan. We're not taking excessively hot showers for extended periods of time either.

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