Underneath your home, you'll see a 10" or 12" flex duct extending down below the underbelly on furnace side and running over to other half of home.
That is your crooss-over duct connecting the two home halves trunklines.
It has most likely come loose, or become kinked/collapsed or has torn open.
It needs to be tight with no kinks or sags and completely off the ground.
Thanks for inquiring,
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