Regarding Quoting someone on the forums

Wasn't sure where to post this.

Can someone explain how to correctly quote someone on our forums here and why is it that when I use the @ Symbol when replying that all the users in the thread do not appear as options, only some do?

I don't understand that functionality.

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Not sure. As Sting and the police sang, "Too much information..."
The @ function seems to work inconsistently. It may link to a member in one thread, but not in another. 

Exactly what I always experience
I get notified often, but not every time. This time it worked for me but not for you.