I've run into this with some restrictive proxies blocking the Marketo domain, but not our web domain, which might be what you're running into. I got around it by replacing the Marketo pod URL with the landing page domain, for example with a landing page domain of pages.mydomain.com:
Embed code from Marketo:
<script src="//app-abcd.marketo.com/js/forms2/js/forms2.min.js"></script>
<form id="mktoForm_1951"></form>
<script>MktoForms2.loadForm("//app-abcd.marketo.com", "000-ZZZ-000", 1951);</script>
Change before putting it on your page to:
<script src="//pages.mydomain.com/js/forms2/js/forms2.min.js"></script>
<form id="mktoForm_1951"></form>
<script>MktoForms2.loadForm("//pages.mydomain.com", "000-ZZZ-000", 1951);</script>
If you are using a Marketo landing page directly you might have to put it in as an HTML block of embed code, instead of putting in the form directly.
HTH