Zoom meeting keeps asking approved attendees to register when they try to join

Does anyone know how to resolve this issue if you've also encountered it? 

Every time I set up a meeting that requires registration (even if attendees get automatically approved) I always have several people who registered but are unable to join the meeting, and are either asked to register again, or cannot join at all because the meeting registration is full. It's very challenging to navigate this when I'm hosting a meeting and getting flooded with emails of people unable to join, and I don't know how to advise them. 

Thank you in advance for any advice!