When a user first registers with the app, they are requested to verify their email, and like everythign else in the world, may fail.
Here are the main issues that should be adressed:
The person does not see the email verification email
Email may be in spam
The most common use-case is the email just being in spam. To avoid this issue entirely, make sure you are using an operational email with good reputation
The user may not exist
The person may have not entered their email correctly, or forgot what mailbox they were looking at.
A quick seach in the the support tab in the system would get you insight, maybe they just don't know with what email they signed up?