Setting up, and the handful of things that usually go wrong.
Email support@closiq.app. Please say which site the video was on and what macOS and Safari versions you are running, which usually makes the difference between a guess and an answer.
That last step matters more than it sounds. Safari does not add an extension to pages that were already open when you granted access, so a tab from before will do nothing until you reload it.
Closiq Sync compares the two videos and will not touch playback unless they match. Different episodes, a different quality URL, or one of you being on a site's mobile version can all read as different videos. Open the same link on both machines.
Closiq Sync works with standard web video. A site using a player that is not a normal video element, or one that blocks extensions outright, will not respond. Tell us which site and we will look at it.
Choose Always Allow on Every Website rather than allowing once. Allowing for one day means it stops working the next day.
That is Closiq Sync waiting for the other person, either because they are buffering or because you have run ahead of them. It resumes on its own. If it persists, one side probably has a connection that keeps stalling.
Safari blocks video from starting on its own if you have not interacted with the page yet. Click play once by hand and Closiq Sync takes it from there.
Reinstalling can switch the extension off in Safari. Turn it back on under Extensions in Safari's settings; your permission grant is remembered.