what Debbie said about following them or them being in the thread.  Plus sometimes I type the @and the box doesn't show up and I have to hit return then space back up and try the @ again and it will work. I try to remember to go delete the line of space I created at the end of the comment.  It seems to happen on the 3rd or 4th person I tag in a comment when I've been "wordy"

You need to be following that person or they need to be in the thread. Sometimes they still will not appear even if you do as Roy Kelley suggests. 

Sometimes you need to type the last name and sometimes you need to repeat the effort to tag members.

The others  have answered the question. 

Good morning Chuck. It works for me, but only if it is a name of someone that I follow.

Chuck Willman - happens to me when I have too many first names with the same in my followers. You can try last name followed by '@' and it should work. In worst case, type a full name and it will show up.

I believe you have to follow them for it to work.

I haven't had too many problems tagging. I sometimes have to do what Tammy decribes and that works very well. I will have to try Roy's suggestion, too. I have one regular follower I can't tag. He doesn't come up. I'll try finding him by his last name. 

Make sure you've followed them or they are the author of the post.

rarely have that issue