The report server aggregates the data collected during the day into daily (and monthly) rollups. This is a scheduled process. If this process is not triggered, you will see gaps in the daily rollups. The most frequent reasons for missing rollups are:
• The report server was down in the night; report data generation starts at 12:10 AM local time generated.
• The report server was overloaded and it took too much time for other crucial tasks; report data generation for long-term reports was canceled.