The Income by Customer Summary Report isn't pulling a few payments received for a specified month

In order to pull the correct sales figures for each  month, I've been running the report Income By Customer Summary. The business sends invoices out, but often doesn't receive payment for weeks, or months, depending on the payment plan arranged with the customer. My issue is that the Income By Customer Summary isn't pulling a few payments received for a specified month. The invoice was created in a prior month, but the final payment is defintely entered/listed in the month specified in the report. So why isn't the report pulling those few payments? Is this a user error, or is there a better report to use? 

(Title has been edited by moderator for clarity)