Why would a Buy/Sell ad not move to the top of the list when you hit BUMP?

I believe you have to wait 12 hours from the last reply to your thread. If you look at the top left above Print Page, you'll see how much time remains before you can bump.

I waited anywhere from 12 to 24+ hours and it still doesn't bump up

If you're referring to the D200 FS post, it was bumped 2 hours ago by Stanicmi.