Notification Hub Issue

Hi,

I face a weird problem when I try to send push notification. I utilize the tags to send push notification. Here is sample registration I have in my hub

{ ETag: '106',
  ExpirationTime: '2014-08-29T22:55:15.823Z',
  RegistrationId: '<regitrationId>',
  Tags: '<userId>,<deviceId>',
  DeviceToken: '<deviceToken>',
  _: 
   { ContentRootElement: 'AppleRegistrationDescription',
     id: 'https://*********.servicebus.windows.net/svmpushnotification/registrations/************?api-version=2013-07',
     title: '*********',
     published: '2014-05-31T22:55:15Z',
     updated: '2014-05-31T22:55:15Z',
     link: '' } }

As per our apps architecture, a user can have multiple device associated with him. So I can have multiple registrations with tab <userId>. But the<deviceId> will be unique across the registrations. Some of my users were not receiving push notifications so to test i utilized the debug feature in the azure portal to send push notification to a iOS device. When I try to send a push notification to tag <userId>  under the results I get three registrations which included the above registration as well and said the push notification were sent successfully. But the user didn't got any notification on every try. So I tried sending using the the <deviceId> of the above registration and it got listed in the result saying push was sent successfully. But this time the user got the notification. What is more weird is this happens only to some users. Can some one help me on finding what might be the reason behind this.