I am trying to wire an in wall timer and am confused about the neutral wire. It's a single pole switch on a multi circuit in a multi gang box. The timer wants me to connect a neutral wire, but when I undo the neutral bundle and connect the one I think is for the light I want on the timer, it doesn't work (the light won't turn on). What am I doing wrong?!
All neutrals stay in the bundle. Add the neutral from the switch to that bundle.
Your wording suggests you are removing the neutral coming from the light and making a one on one connection to the timers neutral. Instead you need to add the timers neutral to that bundle, leaving all neutrals connected together in the bundle.