Pre-paid invoices

I have a Section 8 tenant who likes to pre-pay her portion of the rent at the beginning of each quarter. I received her payment in Oct and applied it to Accounts Receivable. It applied the correct portion to the Oct sales invoice and automatically Nov and Dec. However, in January, when I did the same thing, it applied the correct portion to January but when I created the invoice for Feb, the Sale Invoice shows a balance due in February. It did not automatically apply money to that invoice, yet my balance sheet shows a decrease in Accounts Receivable in the amount that should have decreased.

I think I figured it out. When I allocated the January PrePayment, somehow the Invoice selection was not set to Automatic. I reset it to automatic and the proper amount was automatically credited in February. Thanks to anyone who was planning on trying to help me.