The inability to replicate the problem makes issues like this really hard to chase down. Since they seem to be able to replicate it but you can’t, I would ask them to take a screen shot of the filled out form with “missing required fields” message. It is possible that something, an odd character or something like that, in one of the responses is causing a problem. I saw some issues similar to these a while back with a couple forms but was able to resolve them.
Is the form multi-page or single page?
One other possibility – does the form have checkboxes on it? If so, if a Javascript error happens which prevents the jQuery script which fixes checkboxes so with work with PHP (on the WordPress side) and Python (on the Google side) doesn’t run, the likelihood of encountering the behavior you are reporting is very high.
I don’t think an odd character is the problem; the wrong filled out field with an odd character should still light up, right? I’ve made it really obvious which fields aren’t filled in yet; as it’s a really long form (single page) and that makes it easy to forget fields.
Screenshot
I have used checkboxes. Could I perhaps resolve the problem by making radio buttons or a dropdown of them, or is there also a problem with those?
There is no problem with radio buttons, it is limited to checkboxes and is due to the difference between how PHP (used by WordPress) and Python (used by Goolge) handle them.
What I would really like to see is a screen shot after the error has happened. If you want to fill out my Help and Support Form I can take a quick look at it with Firebug. It is very easy to tell if the Javascript problem is a likely culprit.
I’ve filled out your form!