If there is no data for a given month, the month will not show that data. In the context of the Borrower or Lender Activity Overview Report, for example, a month column will have all zeroes or not show data at all if your library did not process ILL requests for that month.
If you believe this data to be in error, please contact OCLC Support with your OCLC Symbol, the report you're generating, and the month for which you are missing data.