Proxy Accounting

I use proxy accounting to account for how much data each user uses each month.
It works well.
1 problem I have:
You can easily see how much data is being used in the current month.
You can easily see how much data has been used in 2, 3, 4, 5, etc months ago
You cannot see how much data was used last month ie 1 month ago. The table is always Blank.
Hope someone can help.