I am sorry for the troubles. Please make sure Garmin Express is up to date, and if the Forerunner 15 is already added to Garmin Express but suddenly doesn't seem to be communicating, delete the device from Express and Connect then add it back again.
How do I update Garmin Express?
https://support.garmin.com/support/searchSupport/case.faces?caseId={bf0bf6f0-4277-11e2-e8a7-000000000000}
How do I delete a fitness device from Garmin Express?
https://support.garmin.com/support/searchSupport/case.faces?caseId={580cc110-bb9c-11e3-69f4-000000000000}
If you continue to have troubles please contact Product Support by phone.