Check the following:
1. Access the Saved Search on Edit mode.
2. Available for Reminders checkbox must be enabled.
3. Under Results tab > Columns sub tab, check if fields has Summary Types.
Name | Summary Type = Group
Number | Summary Type = Group
Amount | Summary Type = Sum
If fields on a Saved Search contain Summary Types, we cannot add this Saved Search to the Reminders portlet on the Home Dashboard. Users may look and vote for Enhancement 162614 : "Reminders Portlet: Provide ability to acknowledge the Summary Type feature of Searches".