question on Supply trks/depots

[Imp]

Both units (supplier & reciever) need to be stationary for the entire turn for resupply to happen. Unless heavily suppresed automatic as Andy described.
Must be in same hex or adjacent unless a dump or bunker which supply at half rate 1 hex further out.
Ammo Cannisters will not resupply big weapons based on warhead size (see game guide for more info) but the rest should reload pretty much anything.

If you had it by a unit that is firing & it is supplying that unit first it may never get time to reload the other stuff.
If its next to ATGMs SAMs or heavy arty they are slow to reload so it has no "time" to reload another unit further down the list.
Loads in order of purchase so if its main role is to supply arty but might need for SAMs to as a priority buy the SAMs before the arty & it will reload them first otherwise it will just keep toping off the arty as it fires.