Due to changes under macOS Mojave/Catalina, you must now manually allow Disk Doctor access to your data in order for it to scan all areas of your hard drive.
To give Disk Doctor access, please follow the steps below:
1. Open System Preferences
2. Click on 'Security & Privacy' icon
3. Locate 'Full Disk Access' section on the sidebar
4. Click the padlock icon and enter your password
5. Click the + icon and add Disk Doctor to this list by selecting the app from the dialog that appears