None of these solutions seemed to work for us. We have a somewhat unusual situation where trying to notify starting with "@s" doesn't result in any suggestions. As we've got plenty of Steves / Stephens / Stevens, this poses a problem. No other letters suffer from this problem.
Doing an notifying them via their surname (i.e. if their name is 'Steve Fancypants', going for "@f") unlocks them, and you can then get them in that comment using "@s".
Global permissions are set correctly, the emails thing is set correctly, and renderers are set correctly. No idea what could be causing this one.
Help?