Forms authentication: Users logged out before cookie expiration

For some reason my users are logged out of the system every 10-15 minutes or so...regardless of the configuration below....am I missing something?

IIS75:
Configuration for Session state:
    Session state:
        Mode: InProc
    Cookie settings:
        Mode: Use cookies
        Name: ASP.NET_SessionId
        Time-out (in minutes): 1200

Configuration for Authentication:
    Forms Authentication Settings
        Login URL:login.aspx
        Authentication cookie time-out (in minutes): 1200
    Cookie settings
        Mode: Auto-detect
        Name: .MyCookie
         ...