You may have created a Quick Start which you didn't share with all the instances you should have during the creation process. It's super easy to share or un-share an existing Quick Start. Here's how:
PreRequisites
Navigate to Sync Studio. Under the Quick Starts tab, select Quick Start Studio from the dropdown.
Locate the Quick Start you want to share or un-share. Under the kebab menu, select Publish Settings.
You'll see the Quick Start share settings in the right hand bar. In this example, I want to un-share this Quick Start, so I'll click x to remove the instance it's currently shared with, then click Publish.
And that's all there is to it, now you know how to edit an existing Quick Start's sharing rules.