Why is a report missing from the Import Wizard's source picklist?

When you configure an import wizard to use reports, you can choose the folder where your report is stored. We will show a picklist of reports we can use. If your report is in matrix or joined format, it will not appear in the list. Grouped reports also can cause problems with the import. Please set the report to tabular before using it in an import. 

More on import wizard limitations.