I have a form setup for customer reservation information on one of my websites. The form works great, but every once a while I'll get some random spam. I thought I would enacty the verifyimg CAPTCHA to kick the occasional bot. Unfortunately, even though the verifytest.php works, when it's integrated into the form, I get this:

The following error occurred in FormMail :
verify_failed
**********
Error=Image verification string missing. This is probably a fault on the server.

I've tried everything and I read a few posts where some people had success by removing the domain name and limiting it to just "verifyimg.php" in src line, but I can't do that, my form files are in a sub folder. I tried eliminating the domain and leaving the subfolder with the same results. Because the form is on an active site I had to remove the offending sections and leave it as it was, but I'd really like to know, what is causing this?