It is difficult to say without more information. I spoke to the support team and they let me know the most commons issues we see.
1 – Panelists share invitations. If panelist B responds with panelists A’s invitation the invitation manager will indicate that panelist A has responded and panelist B has not. This occurs because each individual invitation is unique to the intended recipient.
2 – The survey URL was truncated and as a result the InvitationID was incomplete or missing altogether. In some cases email clients enforce a maximum line length. Typically the maximum length is 80 characters. Depending on the length of your domain name it is possible that the survey URL plus InvitationID may exceed the maximum length.
3 - The panelist did not use the invitation to navigate to the survey. Depending on security settings all available surveys can be listed for potential recipients.
At this point I think this may be an issue that is better handled by the support team. You can either open an online support ticket at
http://www.prezzatech.com/support/helpdesk/ or contact them by phone at 1 617-715-9600.
To help the support team evaluate this issue they will need a copy of the invitation.