After setting up WHMCS, my Domain checker was not working. Apparently I had over-looked the menu items in WHMCS called "Setup > Products & Services > Domain Pricing". Input the prices for each TLD and suddenly the Domain Checker works! Don't know how I overlooked that WHMCS option. Oops. Makes sense though. How can you sell a domain name if you have no prices set up for it. Just wish WHMCS would've had default prices in there to begin with.
Hope this helped.