[Mobhack]

Do both sides have radios?

is HQ close by (like 3-5 hexes). If not there is a small random chance, even if the non-radio units were 10 KM off. gives a chance for someone a long way away to "get sorted out". Those who like to whine and moan about such things can assume that runners or dispatch riders or MPs got to them. (it was probably put in so that the AI had a chance with elements that had run away a long distance. Humans can pick up "perma-pinned" damaged units with a truck or H/T, AI is too stupid for that).

Is your own company HQ nearby? - your company commander can provide contact, as well as rallying. As can A0 as Bn commander, if close by.

Skilled units, suppression levels. Lots of things determine contact, as well as many random rolls in the various routines.

Off map units go off the net more if the BN HQ is dead or fled, or suppressed. Do not get A0 into combat - this is not SP1 where you only had ~24 core units so making the commander go all Rambo was worth it. A0 (provided you leave him as the default HQ class) is a second class observer - use him as such. In addition he is the "back link" for rear area comms as I just mentioned.

So if you want an HQ to be generally in contact, keep its subordinates close by (within 3 hexes, 5 at a push). You may want to organise your on-map artillery park as a company and keep it tight-packed with the coy commander in the pack as well, maybe put A0 there too. But then again, that would be a juicy counter-battery target...