can't get the session from session store in node.js connect-mongo

I am using connect-mongo to store my session data, blew is the code to start my express server and mongodb session.

var MongoStore = require('connect-mongo')(express);

var sessionStore = new MongoStore({url: config.sessionStore}, function() {
                          console.log('connect mongodb session success...');
})

app.configure(function(){
    ............
    //  // cookieParser should be above session
    app.use(express.cookieParser());
     // express/mongo session storage  //  
    app.use(express.session({ 
                      cookie: { maxAge: 24 * 60 * 60 * 1000 },
                      store: sessionStore,
                      secret: config.sessionSecret,
                  key: 'express.sid',
                          clear_interval: 3600
                                          }));
    ............
})

I have looked into the chrome browser and can of course see my session key- 'express.sid' and corresponding value.

when the user logs into my server, I want to get their session data from my mongodb session store. However, session store query the session data with no error but just showing the output null.

var connect = require('express/node_modules/connect')
, parseSignedCookie = connect.utils.parseSignedCookie
, cookie = require('express/node_modules/cookie');

function fileUpload(req, res) {

    var sid = req.cookies['express.sid'];
    var sessionID = parseSignedCookie(sid, config.sessionSecret);   
    console.log('fileUpload  ',req.ip, sid,sessionID);
    // output the session value as expected 

    // but session store can't query the session data, no error but just no values
    sessionStore.get(sid, function(err, session){
          // access to session
           if(err) console.log("session store err".red,error);
           else console.log("sessionStore auth  ".green+session);
    }); 

    res.send(200);
};

What goes wrong with my node, or any another things I missed?