uCommerce_Payment table not populated

Question

Hopefully someone could shed some light on where we're going wrong here. Our implementation is using Paypal but having set up the integration as described in the docs (http://docs.ucommerce.net/ucommerce/v5/payment-providers/setup-paypal-standard-website-payments-as-a-payment-method.html) attempting a payment was resulting in a plain postback at the "Preview" stage (i.e. click to pay but page posts back and nothing happens).

My understanding is that somewhere in the pipeline the uCommerce_Payment table must be populated for the PurchaseOrder in question and from there the following code (in uCommerce/Preview.cshtml)...

TransactionLibrary.ExecuteBasketPipeline();
TransactionLibrary.RequestPayments();

...should pick up the entry in this table and direct the payment accordingly.

Problem is, when I check the database there are no records in our uCommerce_Payment table.

Any ideas?