First, make sure you wait about 30 seconds. You may also wish to refresh your side and have them do the same as the connection is usually instant or within 5-10 seconds under normal circumstances.
Second, ensure your client is either using Chrome or Firefox or the latest Safari browser. If they are using Internet Explorer or an older version of Safari, please ask them to download and install Chrome or Firefox or update Safari.
Also, double check that they are in the same session. This can happen occassionally if you've created/joined an instant session or accidentally created two appointments for a client. To see what session number you are in, look in the URL bar for something like https://app.thera-link.com/session/######. Those ###### will change for each session and your client should be in the same # as you.
Finally, make sure they check to see if their camera is connecting. Here's a quick place to check all of the settings: https://tokbox.com/developer/tools/precall/
If you've tried those things and still don't see your client, please call for tech support if immediate help is needed. Your client can also call. The number is 1-833-213-0643.