Google docs “published to the Web” can be fully annotated. Google docs which shown in the editor can only accept “Page notes” Note that the URL of the Google doc can depend on the identify of the logged-in Google user.
If the Google URL is of the form …document/d/… then it’s the same for all Hypothesis users. But if it’s of the form …document/u/1/d… in which case they may be annotating a different document than others. Signing out of their different accounts, or using google Chrome in incognito mode, or using the Hypothes.is Via proxy should resolve it.