Not updating session properties in .onDefault

I've a code which is displayed below and the problem is that the userFailedC prop isn't getting incremented. I can set property but its value resets each time user's intent is not matched, i.e. each time .onDefault is executed. Am I doing something wrong? Also, I tried with userData and privateConversationData so I would bet that I misunderstood something. Thank you!

bot.dialog('/', new builder.IntentDialog()
 .matches(/(hi|hello|hey)/i, function(session, args, next) {
   session.send(rm.get('Hi!');
   session.beginDialog('/showMainMenu');
 })
 .onDefault(function(session, args, next) {
   if ( session.dialogData.userFailedC === undefined ) {
     session.dialogData.userFailedC = 1;
   } else {
    session.dialogData.userFailedC++;
   }

   if (session.dialogData.userFailedC % 2 == 0) {
     session.send('Please say hi to me!');
   }
 })
 .onBegin(function(session, args, next) {
   next();
 })
);

via BB04Deng