Do you rack to a secondary vessel at all? If done improperly this can introduce oxygen. Also cold crashing can be a culprit because of the vacuum it creates. I used to have a similar problem but now I dry hop and cold crash in the keg.
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Using a hop bag does reduce utilization according to what I've read. I recently started using a homemade hop spider with a 5 gallon paint strainer bag. It contains all the hops and allows the hops to swim more freely than a smaller hop bag.
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