Do other people have trouble getting the tag feature to drop down and work?

Which social channel are you trying to tag on? It does seem to work better sometimes than at other times. Sometimes I do the tag first before adding anything else to the compose box, and then once I have the tag set, I'll write/copy and paste the rest of the caption in.

Seconding Beth here, it depends on which network you are using but typically making sure there is a space on either side of you @ will help.