Good airflow through disconnected duct.. when i connect it the airflow stops.

I have one vent in my house that doesn't blow any cool air out of it. I went underneath the house expecting to see the duct had fallen off or something... it was all properly secured and wrapped in insulation. I removed the insulation and unscrewed the duct and it blows perfect when it is disconnected and hanging into the crawl space. When i connect it, there is zero airflow through the vent.

I could hold the two sections apart so that there is only an inch or so gap between them, and airflow is normal up through the vent. When i join the sections together, it immediately stops.

Could there be some sort of vacuum or negative pressure (or whatever it might be called) that occurs when the duct is connected to the house, that cancels out the airflow up through the vent?

Thanks for any help!

submitted by /u/whatwhatdb
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