There are a few things that can cause this, especially when Execute Now works but the scheduled report misses one recipient.
I would start by checking the user record under User Administration > Users. Open the affected user and confirm that the account is active, the email address is correct, and notifications are enabled.
After that, go to System Logs > Emails and look for the email created by the scheduled run. Open the record and check whether that recipient was excluded, ignored, or if any error is shown.
It is also worth checking the scheduled report itself. Open the schedule and review the Run as user. Make sure that user is active and has permission to access the report and its data.
If the recipient is an external email address, also confirm that the address was added in the correct email address field.
These checks should help you narrow down where the issue is coming from.
If this helped, could you mark it as helpful or as the solution?
