At my location we get periods of very little CAT activity even on the airways. When I built the first box, tested it, nothing. No traffic showing at all. So I ran up FlightRadar24 to check, and that confirmed there was nothing within 80 or so miles of us.
So I waited until FR24 had a good selection of traffic, both inbound to BRS and on the airway (usually for us late afternoon) and tested it again, and confirmed it did work. But based on my first couple of tests, I could have sworn it wasn't working.
So now when I test I always check with FR24 to see what traffic is about. Also, it may have been said before, and you may have checked, but if you are running SD, you have set the 'Other Traffic' 'show within vertically' in Navigation Options up to say 50k feet so it doesn't filter most CAT out? The default setting IIRC is 5000' which is much too narrow a band for testing unless you are on the approach to Heathrow.