When creating SQL 2014 AlwaysOn Availability Groups, careful attention is required when provisioning the logins on each member server.

While the databases may contain user accounts for the appropriate members, the cluster member servers may not contain login information for said users. This can result in a seemingly “happy” fail over cluster (according to the dashboard in SQL Server Management Studio), but upon fail over, much pain will occur.

From a 10,000 foot view, the Logins on each server need to have the same SID, Username, and Password.

More detail (along with a script to rectify any “on-noes” that may have occurred in your environment): https://support.microsoft.com/en-us/kb/918992

This page contains some best practices for avoiding the described issue: https://aalamrangi.wordpress.com/2015/02/09/avoid-orphan-users-in-alwayson/