Transmission slipping on hard pull on 2001 Buick LeSabre

I have noticed my transmission slips when I go uphill. I have noticed that the transmission is not down shifting and then it will begin to slip. Also my gas mileage has dropped from 24-25 mpg to 20-21 mpg. I have had the codes run on car but nothing comes up.

Asked by for the 2001 Buick LeSabre
Just go down hills