A common problem I experience with users at my company is that people don't realize passcodes are case-sensitive. For instance, lots of people have "covid" as a passcode. Yet people try to join by typing "COVID", which doesn't work because it's supposed to be lowercase. Because of this, I advise against using letters in one's passcode.
It's also best to share the passcode-embedded link with people. When people click on this link, then they don't even have to enter the passcode, so it removes the chance of people entering the passcode incorrectly.
Also, it's not very difficult to accidentally change one's passcode. I would double-check the passcode in both the invite that was sent out and in your Zoom application. You can check on it in Zoom by hitting Schedule from the home screen (or by going to Meetings and then hitting Edit under the Meeting ID you're using), then check the Passcode field to make sure it's still the one you want.
I noticed especially that people who use the Zoom Outlook plugin seem to have their passcodes mysteriously change, so I think the Outlook plugin tends to mess things up--I'd advise scheduling through the Zoom application instead of through the Outlook plugin.