"Image verification could not be verified due to server issues. Please try again later."

I keep getting the subject error message when I try to create a new account on our forum (vBulletin 4.2.3 Beta 3; Ubuntu 10.04 LTS; PHP 5.5.9). Others do not, and we are getting new registrants.Why is the happening "to some" but not "to others"? -- You received this message because you are subscribed to the Google Groups "reCAPTCHA" group. To unsubscribe from this group and stop receiving emails from it, send an email to recaptcha+unsubscribe@googlegroups.com. To post to this group, send email to recaptcha@googlegroups.com. Visit this group at http://bit.ly/1dkFnYd. For more options, visit http://bit.ly/P65DvS.