Hi!
Do you mean you schedule two meetings with the same conference ID?
Can the meeting organizer see all meeting attendees (including the users who met the issue) in the meeting?
In this case, we recommend you try to reset your conference ID for your meeting and then invite the user again to see if it can be fixed. For more details about how to reset conference ID, please refer to:
https://crossthedivide.zendesk.com/hc/en-us/articles/360013337191-How-to-Reset-Your-Skype-Conference-ID