If your FedEx Ground shipping is not working on your site, please verify that you do not have the "Residential Rates" box checked in Preferences > Shipping > Shipping Methods tab > [FedEx Ground shipment method name]. If you would like residential rates for FedEx Ground, you need to select FedEx Home Delivery as the service.
You can also verify that this is the cause of your issue by going to Sites > [site name] > Settings > Shipment tab. Scroll down to the "Test Shipping Options" area. Enter a weight and a postal code and then click the "Test Now" button. If you see an error message that says:
"Shipment not possible ERROR:  This shipment qualifieds for FedEx Home Delivery. Please resubmit your request with service type Ground Home Delievery"
Then the residential rates box being checked on a FedEx Ground shipping method is the cause of the error.