2001 Gmc Sierra C3 Transmission Problem

I own a 2001 GMC SIERRA C-3, same as YUKON denali motor and tranny, i would say in the last 6 months i have noticed that on my typical drive home the truck just isn't down shifting, normally it was fantastic to always be in a nice power range and never hesitate to down shift while passing or even slightly speeding up, now i get on the highway and even excellerating from 35 mph it won't downshift i have to use the selector and put it it into 2nd and shift back into drive, it seems very lazy to react, if i put it to the floor it sometimes down shifts, is there maybe an update on programming the tranny to be more precesise.

Vortec 6.0 L VIN #11209158