You can be under a traffic service but the controller is under no obligation to pass you details of any traffic whatsoever, if he is busy. I have had many encounters with such unreported traffic.
It's possible that traffic is not displayed for whatever reason, but if that is the case the Controller 'should' either reduce the service due to workload with the reason, or decline the service offering the best alternative and reason.
But we are now moving into the circular ATSOCAS debate.
Whether or not a controller is giving a better or worse service to the traffic he/she is working from your experiences (which I don't dispute). The service requested is more important to the ATCO than the flight rules as it dictates provision of service.
