There's specific conditions that are met that determine weather the message can be resumed or not. I know there's a blog site on it but I cannot seem to find it. Here's other resources. And remember that adapters all have different conditions as well. I know the HTTP can be tricky with resumes.
Look here: http://blogs.conchango.com/matthall/archive/2004/12/16/466.aspx
http://bencops.blogspot.com/2006/05/biztalk-2006-notes.html