Sagging flex can choke the liner before the air ever reaches the diffuser. Re-support the run so the duct is smooth, not crushed, not kinked, and not stretched into a bad bend.
Do not leave flex hanging like a hammock, cinch straps until the liner is crushed, or hide kinked flex above ceiling tile.
Check
Look for sags, kinks, crushed straps, torn jacket, or inner liner pulled loose at either end.
Check that the flex is not longer than needed and is not snaked around obstacles for no reason.
Make sure bends are wide and smooth instead of hard folded.
Verify the inner liner is attached and sealed at the collar, not just the outer jacket.
Check project/manufacturer support and bend requirements before calling it corrected.
Steps
Look for sags, kinks, crushed straps, torn jacket, or inner liner pulled loose at either end.
Check that the flex is not longer than needed and is not snaked around obstacles for no reason.
Make sure bends are wide and smooth instead of hard folded.
Verify the inner liner is attached and sealed at the collar, not just the outer jacket.
Check project/manufacturer support and bend requirements before calling it corrected.
Say this to your foreman
The flex run to [room/diffuser] is sagging and the liner looks restricted. I can re-support it, shorten the ugly slack, and clean up the bend path per the approved method. Do you want that corrected before ceiling close?