Whitelisting the IP which resolves to your whmcs domain is not enough if the Hosting Server have multiple IP's. You should whitelist also the Hosting Servers primary IP.
Download the Server IP Check script from our FREE Download section: Server IP Check
Upload the script into the whmcsroot folder and access the same with your Browser. The displayed IP must be whitelisted as well as the domain IP itself.
Once whitelisted the IP's, goto the FREE Tools Home and reload the account settings by selecting the reseller account from the dropdown menu "Change to:"