I have found the reason why this hook was not working on my board.
I had -RAW33- New topics 1.1.0 installed on my board and after disabling it, this 'group format' hook did actually work.
So both hooks inflicted each other.
On all the skins I have its not working properly. The skins are all purchased from Evanescence at IBSkin.com.
Even on the default skin of IPB its not showing on forum view. Just like on some of the other skins.
I have just installed this hook and it appears that its not working on the skin that I'm using. I have a few skins installed on my board and on some of the skins it's showing but only in the board index and not in the rest of it.
I'm running ipb 3.3.1 and I have checked if the skins that I'm using are are updated and they seem to be.