I personally use:
CODE
$if(%tracknumber%,%album%%discnumber%,!noheader!)
Again, try without the dashes.
As Hakubo indicates below you may want to look at your sort string from either:
File> Preferences> General> "Sort incoming files by:"
-or-
Edit> Sort> "Sort by ..."
SCPL will group by whatever you have listed in the group by field, however it will not rearrange your tracks to fit those groups. You must first develop a working sort string to place the songs in the order you desire before you can achieve a desired grouping appearance.