Puzzelled can you help

Hi all, I am wondering if I can get some ideas as to why a Military flight that I saw with my own eyes and on looking at my adsb was not showing it, I relise that milatary flights are not generally posted, the aircraft I saw as a apatche helicopter I have seen some milatary flights over Scotland, hence the question is it a setting on my account or is this just the norm…
Thank you in advance
Derek G1GDB

Need MLAT - https://flightaware.com/adsb/mlat/

hi mlat is synced with 169 stations

Two main possibilities:

  1. He had switched off ads b.
  2. He was at too low an altitude to be seen by the required number of MLAT stations.

Not all military aircraft are even using Mode S which is required for MLAT. In theory we will see more military ADS-B adoption as we get closer to January 1, 2020 in the USA.