EWS Pull Notification problem with unread mails arrived between polling intervals

EWS Pull notifications are great in fetching new mails arriving while polling for the notifications, however if I increase the intervals between polling reque开发者_JAVA百科sts, the mails arriving between each requests do not get picked up. Am I doing something wrong, or missing something trivial?

String subscriptionID = null
do {
  PullSubscription subscription = service.subscribeToPullNotifications (...subscriptionID ...) 
  subscriptionID = subscription.getWaterMark();
  ...

} while(true);

Retaining previous watermark solvEd it