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Retest the form in IE and Firefox, and get someone else to do a dummy test also to be sure.
Then look at your traffic stats to see if the page with the form on it is getting hits. And again compare that to the hits on the form handling program. If your form page is getting hits, but form program is getting no hits at all, that means people are seeing the form but just submitting it, which could be strange.
You might also see that the form page is getting hit, but that the form program is generating some kind of error (see error_log on Apache). And maybe it's an error that only happens when visiting your site from somewhere else.
Nonetheless, use the above method in any test you do from any location. Watch your access log and your error log. Hopefully that will help to reveal what's going on.