One possibility is that each time you subscribe, you set the Subscriber element to the same div. If you are manually creating a div inside the streamCreated handler on the session object, make sure that you are specifying a unique div id for each stream. A common technique is to append the streamId to the name of the div to make it unique. Inside the streamCreated event, you can obtain the streamId as event.stream.streamId.
It is also possible that you are not subscribing to streams when you think you are. Place a log statement in your 'streamCreated' event handler that increments a counter for each Subscriber. This way, you know how many streams you have actually subscribed to.
If you can hear the other Subscribers but not see them, you probably have gone into Audio Only Fallback. See: How do I know if I have gone into Audio Only Fallback mode?