Applying credit to customer invoice in Accounts Hosted

I'm having trouble finding / applying a customer's credit in Accounts Hosted, and I'm hoping someone can help. 

I have a customer with multiple jobs, and invoices against each job.  I received a payment for an invoice, but they paid too much.  To receive the payment I selected the customer (not the job), new transaction, received payment, ticked the invoice to pay, then opted to 'save credit to be used later'.  Now the customer has another job with another invoice attached.  They paid the balance owing (allowing for the previous credit), rather than the total of the invoice, and I received this payment the same way as above.  When I try to apply the previous credit to this new job/invoice, it does not appear in the credit list.  The customer's balance is now correct at zero, but the credit is still floating somewhere and the last job still has an amount owing equal to the credit.  I've obviously not married the two up correctly but I'm clueless as to how to fix it.

Any advice would be very much appreciated, thank you.