How to Add Fields to a Custom Report Type

The following steps help you add missing fields to your report:
 

1. From Setup, enter Report Types in the Quick Find box, then select Report Types.
2. Select the name of the Report Type you're using. 
3. Click Edit Layout.  To find additional fields, search for fields in the search bar or you can also select option to show available fields.
     
4. Drag and drop the missing fields from the Fields section, then click Save.
5. You can also add additional fields via lookup on your primary or secondary object by clicking + Lookup Fields.  

 Note: If previewing the layout, all fields and objects are displayed, including fields and objects that you don't have permission to access. You can access only the data that is stored in the fields or objects that you have permission to access. 

When you go back to the report, you will find that the fields are now available to be added to the report.

Create a Custom Report Type