I found the disco Skelly last night and again just now but I am not receiving the achievement when I go to my achievement page. Am I missing a step somewhere?

I have gotten my achievement now but it took me 4 times of finding it and uncovering before I got it!

I wonder if some of the 'special skeleton' achievements only register if you get to the bottom? I'm honestly not sure though, I wasn't paying any attention when they registered for me.

You can definitely get htem without going to the bottom. You do have to open a new tab/go to another page and refresh before they register though. And you have to make sure you uncover the skeleton completely; if there's a coffin over part of it you have to wait to find it again