Form Submission Inconsistent

We have a basic CGI POST of a form in our course. It submits to a PHP page and returns the students transcript ID. This works for a large number of users. However, some users can't seem to submit the form and the returned value gets listed in the SCORM variables as ~~~null~~~. The form submits to the same server (no cross browser) using a relative path.

Is there something in a user's browser settings or firewall that would prevent a simple CGI post and returned value from working? I can't replicate it but I have users who definately have the issue.