This could be happening for a couple of reasons. Each video meeting, whether scheduled or not, has its own unique meeting ID and passcode and cannot be used with another patient's consultation. This is for security reasons, so there is no worry about another patient attempting to access their's or your personal meeting at a later date or time. All invited participants simply click a link in an email to gain access to that specific meeting ONLY. The host is the ONLY one who has the ability to accept or decline access to you and all other participants.
The other reason that this may be happening is that an Admin or yourself has already been in the meeting & left it prematurely. If this is what has happened, the video meeting URL will break and the meeting ID & passcode will no longer be usable, and another meeting invitation will have to be created. Again, this is for privacy and security reasons and will make the meeting that much more secure when it commences.
To avoid this from happening, do not enter your video meeting until close to your scheduled appointment time (a minimum of 10 minutes prior or less), and do not leave the meeting until you have finished speaking with your Healthcare Provider.