Issue with selling to other countries

[boxist2023]

I have an issue with my markets, I have set up the shipping and delivery for the Countries we wish to sell to. I have set up the delivery costs etc, all seems fine until the customer goes to check out they then receive the follwoing message 

"Shipping not available

Your order cannot be shipped to the selected address. Review your address to ensure it's correct and try again, or select a different address." 

I have had customers message us to tell us they are unable to checkout making us lose sales. Please would it be possible to provide some guidance as to potential solutions.