If you are using Microsoft Teams Rooms (MTR) then you may run into issues where your http://dearmckenzie.com/wp-booking.php external meetings are getting rendered without the join button.
On the MTR table panel it will look like this where meeting will be displayed but there is no Join button.
This is likely related to your organization using some sort of url defense system which automatically modifies the external URLs.
For your MTR devices to be properly parse the external meeting you are required to white list the Teams, Zoom and Webex URLs.
Once you whitelist the above domains you should see that external meetings are getting rendered without any issue.