If you watch the regulator cable while you are pushing up and down on the window, you will see that it is not taught. This is how I fixed mine 8 years ago. This window still operates perfectly after installing the bicycle brake cable adjuster. Takes about 30 minutes to do, including removing the door interior panel. No need to pull the regulator out of the door to install. Photos show the passenger side regulator.
Window regulator permanent fix for stretched cable