Our school district is made up of at least 7 different Google domains (school.org), one for each school and one for the central office. While this makes it easier on the Google Administrators in the district, it causes some issues when it comes to sharing between these domains.
One issue that has come up recently is the sharing of Google sites. Teachers are not able to view sites shared with them by a teacher in another school or at the district level in their "sites" app.
If you are having this issue, the easy solution to this is to go to https://sites.google.com/site/sites/ for a list of all personal sites that have been shared with you OR https://sites.google.com/a/domain.org where domain.org is the domain of the person who shared it with you.