PDF Invoice not being produced

Hi
I purchased your PDF invoice add on to go with your shop styling plugin and I have now noticed that it is not producing or attaching pdf invoices when the order is successful. If I click on the preview button it brings up the last invoice that it produced. I have had a search of the forum and tried to identify errors, the only thing I can find is that the directory /wp-content/uploads/wpsc-invoices/ has its permissions set to 775, should this be set to 777? Also is there anything else I can do to diagnose the problem?

https://www.ads-software.com/plugins/wp-ecommerce-shop-styling/