Bec,
Are your non-marketo pages built with the Munchkin code? If yes, you should be able to use the webpage constraint and type the webpage URLs directly into the constraint choice window. If the code is not there, adding the code should help you in this regard.
This is a common cross-domain cookie scripting problem. Without the Munchkin code (marketo's cookie), all the visitors are going to the non-marketo page and Marketo cannot see any of these page visits until the form is completed. In a sense, this is the first time Marketo "sees" a lead action on the page. And since the form is on several different pages, it can't determine from which page the lead is coming from. Proper use of the Munchkin code across all of your pages will allow you to use the same form on multiple web pages and sort by which page was visited prior to form completion.
Good luck!
-Ken