Why log out sooner than ExpireTimeSpan in asp.net identity?

I am working on an asp.net mvc application that used asp.net identity.
In Startup.Auth.cs file I set ExpireTimeSpan to 20 days but when I log in to my app, sooner than 20 days my app is logged out and I have to log in agian!

Startup.Auth.cs

app.UseCookieAuthentication(new CookieAuthenticationOptions
{
    AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,
    LoginPath = new PathString("/Login"),
    Provider = new CookieAuthenticationProvider
    {
        // Enables the application to validate the security stamp when the user logs in.
        // This is a security feature which is used when you change a password or add an external login to your account.  
        OnValidateIdentity = SecurityStampValidator.OnValidateIdentity<ApplicationUserManager, User>(
            validateInterval: TimeSpan.FromMinutes(0),
            regenerateIdentity: (manager, user) => user.GenerateUserIdentityAsync(manager))
    },
    ExpireTimeSpan = TimeSpan.FromDays(20),
    SlidingExpiration = true
});

And in Login action:

var result = await SignInManager.PasswordSignInAsync(model.Email, model.Password, model.RememberMe, shouldLockout: true);

Update
When I log in, .AspNet.ApplicationCookie is generated and it's expire date is set to "20" days later, And when I open site next day, I am logout but the cookie is exist.

What is the cause of this problem?
Thanks in advance.