I had a bit trouble with tagging people in chapters not too long ago. I know how to tag people in comments and stuff, so I was really irritated and confused when the people I tagged in a chapter was not showing as a clickable link as usual. 

Turns out, you can't just go to the person's profile and copy the whole username along with the "@" because that's exactly what I did. Being the lazy person as I am, I thought I could just copy and paste, but no. You can't. You have to type "@" before typing or pasting the username. So lesson learnt, don't be lazy.

flamingogirl102, izzywriter, MilaLoves2read and icypenguin17, I would like to thank you all for helping me figure out this pretty dumb mistake. So, thank you! :)