This may be due to your row limit. If so, increase the "# of rows to fetch" setting on the "Split by" tab:
Another issue is that the data source may not return dates for which there are no metric values to display. For example, if you fetch conversions by date, AdWords may return only dates that had conversions. If you want to always get all date values, type "SHOW_ALL_TIME_VALUES" to the Advanced settings field in the Options tab of the sidebar. For existing queries, you can add it to the Special settings column in the SupermetricsQueries sheet.