Apigee BaaS expired token

Question

I have an app with several users who log in on a regular basis to do Q/A on my data. Every single one of them can no longer log into the app. When they attempt to log in they are getting an expired token error. Here is my code:

client_creds = { orgName: "org", appName: "app" };

dataClient = new Apigee.Client( client_creds );

dataClient.login( username, password, function (error,response) {            
    if (error) {
        //error — could not log user in
        alert( error.message );
    } else {
        //success — user has been logged in
        token = dataClient.token;
        initializeUI();
    }
} );

I don't know if users are actually logging out or not. In any case why are users getting an expired token error? How can the token get refreshed on login?