Has anyone else had any problems with settlers not being willing/able to go to their assigned tasks if it's more than two stories off the ground?

I've got a bar set in up in Hangman's Alley that's three stories off the ground and the settler I've assigned to it won't go up there.

I don't have the problem when the bar is on the second story.

No.. I haven't had that issue. Most of my stores are pretty high up. I have a 9 story complex going on with stores all over the place and they're all manned reliably.

Make sure they can easily get up there. If you cant then they can't. Play around with access ladders.