Hi @gubbins,
I've just had a thought but will need you to test it out to see whether this might be your issue...
Can you go to your "Town Square" channel and
1. type @
in the center channel message box
2. in the search box type in:town-square from:
Can you compare the list of names that appears for 1. and 2. above and let me know whether they're the same?
I think the issue you might be experiencing is that typing @
in any channel only brings up a list of channel members whereas typing from:
in the search box brings up a list of all users on the team.
Let me know how it goes.
Thanks!