Supply truck question

[warriorsq]

I just came across a demo of this game by accident and am really enjoying it, but have a question. I made a couple of Greyhound armoured cars and after a while I realised I needed to re-arm the 37mm HE shells as they were running low on at least one of the Greyhounds. I made a resupply truck and drove up to the armoured car, went to the trucks inventory, clicked on the shells and moved the cursor over to the Greyhound and the car lit up, the cursor seemed to change to a little kind of downwards arrow that I thought meant I was doing the right thing but the ammo just ended up on the floor...what am I doing wrong?