I know this post is a little old, but I had the same problem and figured out how to fix this, so hopefully it will come in handy for someone else (at least it worked for me):
You have to make sure the track you want to navigate around with the arrow keys is toggled on (click on the little V1, or A1, or whatever track you are working on, on the left side of the timeline). I had accidentally toggled the track I was working on off somehow (I’m sure it’s easy to do since it’s just a click to toggle on or off), so I toggled back on and it is working again. Hope this helps someone!