2005 Chevrolet 2500 HD, Express Power Window

Can anyone explain how the express up/down systems work or in this case fail to work completely on an 05 Silverado? Drivers and pass side fail to express all the way up and stop at varying places. Getting them to roll all the way up requires continual flicking of the up switch. I was told a dirty track would cause that but.... I tried reprogramming both windows using the switches with key on and key off....didn't help. Previous owner of the truck said he added that feature to the truck.....not sure if that's a door switch replacement or not....just want to understand how it works so I know what to fix.