Best guess is blockage in the ducts.
The question is is this a new problem that just started?
Best / cheapest option is a snake cam with a light that hooks up to your cell phone.
I bought this one for myself on Amazon for $35.
Here is the link....
https://www.amazon.com/DEPSTECH-Wate...NsaWNrPXRydWU=
Lots of uses and good to have for home, auto and RV. It has a built in light. If not at least you will know if it is clear or have a visual on what and where the blockage is so you can figure how to deal with it.
Good luck!!!