Have you created a new Custom Field but it doesn't show up in the Report Builder? You may need to create a new Report Type and use that to build a brand new report. Instructions for creating new Report Types can be found here: How To - Create Custom Report Types with Custom Fields
Luckily in many cases, you can edit the existing Report Type to add your new custom field. This Article covers how to identify if the Report Type you're using is Editable, and if so, how to Edit the Report Type.
NOTE: If the Report Type you are using is Managed, it will not be Editable.
Where can I find the Report Type used for this Report?
Click "Customize" on the Report and then look in top left of the Report Builder tool. You should see the Report Type listed here:
Find the Report type in Setup
Navigate to Setup
In the Quick Find, search for "Report Types"
In the Create menu, click on Report Types
Is the Report Type Managed or Unmanaged?
From the List of Report Types, you will see an icon that looks like an envelope with a blue arrow next to any Managed Report Types.
NOTE: Managed Report Types cannot be Edited. You will need to create a new Report Type which queries the same objects. See also: How To - Create Custom Report Types with Custom Fields
Example: Of the Report Types shown below, only Loans and Lead Sources with w/o Appointments with w/o Sales Report Types are Editable. The other Report Types shown here are Managed (as indicated by the icons).
If Unmanaged, Add Your Custom Fields to the Report Type
Next, click on Name of Report Type you'd like to Add Custom Fields to
Then click the "Edit Layout" button
Drag and Drop the Custom Field(s) into the matching group of fields for each Object
NOTE: Choose from the View drop-down menu to select a different Object in the Report Type and add Fields from other Objects. Be sure to drag-and-drop the fields into the appropriate Object sections so that they can easily be located in the Report Builder tool.
Click "Save" and then return to your report.
You will need to re-Run your Report again, for the changes to take effect. If done correctly, the Field(s) should be accessible in the Report Builder tool!