Repairs Cub Cadet 1554 SLT transmission issues

I have a 1554 that I am really starting to have issues with the transmission. When I get to any type of hill, the tractor stops, moves just a little then takes off as if nothing is wrong, its starting to do it more and longer before it takes off. I have checked and replaced the drive belt, drained the fluid in the transmission and nothing is helping.

Thanks for any help you guys can provide.