The most common reason that using the @ and # symbols does not translate into actions that are recognized by our system is because students are copying and pasting content from another text editor into the Yellowdig Post editor. The @ and # symbols, in particular, have to be typed into the editor window to be recognized "on-the-fly" as you are typing. This prevents these and some other symbols from being incorrectly recognized if pasted in as part of a formula, for example.
The easiest fix for most symbol recognition and formatting issues is to compose your Posts entirely in Yellowdig's Post editor rather than copying and pasting. If you're worried about losing work due to a technical problem (e.g., your computer crashes), write your Post/Comment in a separate document, copy-paste the content into the editor, and then find and replace each @mention and #hashtag so that these symbols are recognized.