Jim:
If you've got a customized form, then it's highly likely that the markup the form is generating isn't what SPCascadeDropdowns expects. On the other hand, if you enabled debug mode, it ought to be complaining about things.
Check out this blog post for some basic steps to ensure that everything is set up properly up front:
http://sympmarc.com/2011/07/08/adding-jqueryspservices-to-a-sharepoint-page-step-one-always/
M.