If you look at the Order ... what is the Order Status set to?

Downloads, by default, are available immediately if the Payment Module is configured to set the Order Status to Processing (orders_status = 2) or Delivered (orders_status = 3) or Updated (orders_status = 4) ...

If the Order is set to Pending (orders_status = 1) and you change it to Processing, Delivered or Updated ... then the Download button should appear ...

If not, if you look the Product Attributes for this in the Attributes Controller ... do you see a green dot next to the filename for the Attribue(s) on this Product?