'Opportunity Pipeline Trend' report does not show all weeks in a month

The Time Frame listed on the Opportunity Pipeline Trend reports will usually be set to the As of Date Interval under Opportunity Trend Information. You will note that when the report is customized, and the As of Date is grouped by Calendar Week, the report will only show the first week of each month.

This is by design and there is currently no way to have all weeks in a month show up on the report.

The As of Date field lists your opportunity data by date as of the beginning of each month for the date range specified. Remember that As of Date calculations are in Greenwich Mean Time (GMT), not your local time, and will display one extra interval if the GMT date is the last day of the month.