Google Analytics Ecommerce Tracking

I have GA installed via Settings - Advanced Settings - External API Key (and GA's ecommerce setting switched on)

When in GA I can see all activity, but it is not showing ecommerce data.  GA is seeing approx. one in 5 transactions.

Has anyone else had the same issue?  I have read several posts referencing the issue, but none seem to be resolved.

I have no idea what this might be but what's the payment processor? I have seen so many times problems like this being caused by the fact that on 3rd party processors once the order has been completed, such as PayPal, the user closes their browser and does not go back to the website. This means that the system never captures the order leading to discrepancies.