WCF server component getting outdated user name

Question

I am overriding System.IdentityModel.Policy.IAuthorizationPolicy.Evaluate as follows:

public bool Evaluate(EvaluationContext evaluationContext,ref object state)
{
    var ids = (IList<IIdentity>)evaluationContext.Properties["Identities"];
    var userName = ids[0].Name;

    // look up "userName" in a database to check for app. permissions

}

Recently one of the users had her user name changed in Active Directory. She is able to login to her Windows box fine with her new user name, but when she tries to run the client side of our application, the server gets her old user name in the "userName" variable in the code above, which messes up our authentication (since her old user name is no longer in our database).

Another piece of info: This only happens when she connects to the server code on the Production server. We have the same server code running on a QA server, and it does not have this issue (the QA server code gets her correct (new) user name)

Any ideas what could be going on?

Edit: They rebooted the Prod server and the problem was resolved. Still would like to know what was possibly going on and if it could recur...