Does the Date User Filter in your Dashboard leave out a few reports from filtering? This happens when the reports are not created over the same table and the (time) column over which the user filter is created.  

To solve this, Zoho Reports has a Timeline User Filter. This filter applies the criteria on all reports, matching it with the best possible date column associated with a report. i.e., if your reports are based on a date column, then it will filter it for that data column. 

To know more on how to create Timeline User Filter,  refer here.