I am still using 10.0 but I'm sure I had this problem several years ago.
When you use ArcGIS Project Center make sure you are using the fully qualified domain name (FQDN) of the server.
Before adding the service to your project, you need to type in the name of the server of where your services are located.
In some cases you need the FQDN depending on how the server is setup.
An example without FQDN: MyGISServer/Arcgis/mobile
With FQDN: MyGISServer.division.local/Arcgis/mobile
I hope this helps.