Where can I see the next collection attempt that will be made for a past due invoice?

If your customer has an invoice in the past due state when you go into the invoice within their account on the left-hand side you will see the Collection Info table. In that table, you will see the date/timestamp for the Next Collection Attempt which will try to collect against the associated billing information on the account. You also have the option of using the Attempt Collection to try and collect prior to the date/timestamp given there. You will want to keep our maximum retries guideline in mind when considering using this option. 

Also, keep in mind that you will not see this option for manually collected invoice. Those invoices are specifically intended to record payments that are received outside of Recurly. Therefore, by default, there would be no payment information to attempt a collection against. However, if your customer wants to make a payment against a manually collected invoice using for example a credit card, we do provide an option for this as outlined in our documentation here.