Nintex "newform" ignores custom master page and goes to the wrong places

I'm not sure what's going wrong, or what else I need to do, so I'm hoping someone here has seen this (and better yet, knows how to fix it):

 We built a fairly ordinary form for a list in a site that has a custom master page.

However, when we try to add an item using this form, what we get is the form displayed in a system master page ("seattle", of course) AND a ribbon, neither of which we want. 

 Worse, both Save and Cancel buttons go to places other than where we want them to go.

Thanks in advance for your help

Josh