Dice roll functions

I can't get the dice roll functions to work.

In what context can dice roll functions work? New posts in a thread? Replies?

What is the syntax for the roll commands? I'm using:

[ roll ]3d6[ /roll ]

But instead of executing a roll function, I just see what I attempted to type as a command as text. 

8

[roll]1d20[/roll]

 Works fine.

The top line was 1d20 using roll tags. The second line was me using the "insert code block" button so I could show the roll syntax. 

Works in all posts, perfectly. So I must assume you're not doing it correctly.