Have you changed your cabin air filter?
In my 2014 it was real dirty after 2 years. I now change it every year.
Also as @Mark_ mentioned it could be icing. Also a poor air flow because of very dirty filter can cause icing further complicating air flow. It can also cause mold!
There are threads on this forum/you tube on how to do the filter change. I do mine now in 10 minutes.:smile: