Are the users trying to forward attached messages? The CommandMessage property can only be expected to work reliably in GroupWise 7.0.3, which adds support for attached messages with CommandMessage.
In addition, here is an excerpt from the online documentation:
CommandMessage isn’t available for new messages. If you try to access Manager.ClientState.CommandMessage from a new message, GroupWise 6.5 throws an “Unspecified error” exception.
I hope this helps.