Hi Andy,
Aircraft need to be ADSB equipped to show on the map as amongst the information given by this equipment is positional data. Most smaller stuff, biz jets, military and some airliners such as Flybe's DHC8,s only have mode-s equipment so while we can pick them up and know that they are around they dont give off their positional information as ADSB equipped aircraft do, so cant be shown on the map.
Alan