What version of MAS are you on?  When you do month end close it updates the YTD and PTD figures and updates the tables the Monthly reports are based on. This is why the report is off.

Thanks! I got it. 

BTW, do you know if there is a way to show each item number on this report? The standard report shows only the summary of product line. Thanks. 

Perhaps the Inventory Sales Analysis report will give you what you are lookign for.  This is in the Inventory Management module rather than the sales module.

Depending on how many items you have, this can be quite lengthy so you may want to preview the report first or print to pdf to see how many pages it will be.