Whole house fan timer issue
Your timer is a spring wound timer that is simply getting stuck. Replace it.

It’s also possible that it’s not screwed together quite right, too tight or too loose either one can cause the knob to bind on the plate.

Also if you’re setting a short time period under 2 hours, turn well past 2 first (to wind the timer) then backup to the desired time such as 1 hour.  (That instructions once existed on the installation/operation instructions but it seems Intermatic no longer specifically details that on the printed sheet.)