Generally, the favorite location screen appears after the sign-up process is completed. When signing up via email, just after the signup, the favorite location screen appears whereas, in facebook signup, the favorite location appears after the verify details screen.
So, the only generic reason that they don't have a favorite location can be that after signing up guests might have closed the app without updating the location in the app.
This can be cross-checked by looking at the user timelines without any activity.
Guest timeline= Edit profile= Favorite location.
The guests who have completed their profile, has a location on the timeline else It will be blank if profile is not completed.
If further assistance is required, submit a ticket to Punchh Support. (For help submitting a ticket, click here)