Gerard, basically a/c fall into four categories as far as SBS-1 and AirNav's RadarBox owners are concern. Fully equipped ADS-B a/c (which transmits full data incl lat & lon), partial ADS-B a/c (full data but no lat & lon), ModeS a/c (some data but no lat & lon) and non equipped a/c.
With full ADS-B we are able to see and plot their location. The second and third type of a/c we can "see" that they are within range but their location cannot be plotted because their lat & lon is not transmitted.
As you know PlanePlotter is feed by data received from SBS=1/RadarBox owners from around the world and can only display actual plots when lat & lons is available. There is an add on to PP called MyCircles which estimates where non position reporting a/c are but that requires a number of feeders seeing these a/c like a form of triangulation.
The only ADF a/c currently equipped with full ADS-B are the RAAF's VIP B737 & the Army's 3 Kingairs. The C17's & C130J's fall into the type 2 category while some C130H's, VIP CL60s and the School of Nav Kingsairs are only ModeS equipped so they unfortunately don't show up on PP. Likewise you will won't see visiting USAF or RAF a/c.
Other a/c like the Qantas' Dash 8s and Virgin's Embraers are only ModeS equipped as well as some older B737, B767 & REX's SAABs. However some of REX's are slowly being upgraded and are now displaying full ADS-B.
Each Australian registered a/c is allocated a hexcode. Starting with VH-AAA = 7C0000 to VH-ZZZ = 7C822D and when (or if) they have either ModeS or ADS-B equipment installed uses that hexcode to i/d themselves to the system. Unfortunately a small number of a/c display the wrong hexcode like VH-XSG (finger trouble) or are still using their country of origin hexcode like VH-DTO, however, when the PP feeders from around the world unite and come to power, we will sort that lot out
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Hope that helps to explain why you don't see certain types on PP.