applying a payment to an invoice through bank feeds

hello, I am a very very new user to MYOB and this has stumped me. Our customers get invoiced an annual fee and then make monthly payments to pay that off. WhenI go into bank feeds I can receive the payment and apply it to their invoice. I've since learnt that there is a rule that advises possible invoice match and I am using that. I have one customer who went into credit on her last payment, and this month's payment is entirely a credit. Our practice is to keep the account in credit and apply the credit to the next year's invoice. When i go into bank feeds and hit receive payment, nothing is happening. The invoice doesn't link and i can't apply the payment in any way. Once I enter her details the amount applied bit that you usually fill out won't appear. I have tried clicing closed sales and that's not working. I have tried other ways but the payment doesn't then appear in her ledger. I am sure i am missing something basic but advice please??