When I send an invite from iCal, one email invite is getting sent to this invitee

Thank you for the checklist. I have tried all of the suggestions and everything is fine with the email address, contact card and internet link. When I look in iCal invitation on my Mac, the email address in the Invitee section has a question mark in a circle next to it which the symbols list equates to : The invitee hasn’t responded to your invitation, but they are available during the time requested. But I haven't received the invitation in the inbox for the email address in question. I added another contact and received the email, and was able to accept it and now have a green tick in a circle. Could it be a problem with iCloud? Thanks for your help.