Sharepoint Some Side Bar Links won't drag into sub-links. Why?

I'm not sure how to describe this issue but I have a seemingly intermittent problem when editing links on the side bar of my sharepoint sites.

Basically I can't make them tree under an existing link. Is there some setting that stops this from working on specific links?

To be clear, in the same side bar I have links that work perfectly fine when inserting them below an existing link.