I had the same problem, and I noticed an almost identical conversation going on over here:
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PZmOAAWBasically, I contact support and they said unless the page the form is on is an official Marketo landing page, the "Filled out form" with constraint "On page" will not work.
WIth the help of another community member, I managed to come up with this work-around that worked for our environment:
Instead of using the "On Page" constraint, I switched to using the "Referrer" and using the "Contains" logic, and then simply plugging in the URL of the page with the form of it (minuse the http:// nonsense).
Be aware, your mileage may vary. The main reason (that I can tell) that Marketo is counting the page the form is filled out on as the "referral" page is because we basically rip out the html from marketo generated forms and we don't actually use any of the normal JS that goes along with Marketo forms. This solution may/may not work if you use more complex forms, or if you use them in the officially supported way.