How do I improve my Gnome's armor skill?

I have worked up a training program that allows me to run a military where I can let a couple squads train up to fight the incoming riff raff. In my two squads currently, for each gnome I have between 150 and 250 fighting skill, 120-150 dodge, 100-130 shield, and I am currently working one squad in hammer, and the other in axes. (I like axes...)

These two squads are front liner fighters and by necessity need armor and in this case, I have them fitted with bronze sets. They both have been training, the oldest squad for 5.5 years of 6 and the youngest for 4.5 years of 6. However, the armor skill has yet to raise for any of them, despite the use of the bronze armor. The older squad has been using leather armor for 2 years and for the last year, it has used bronze. Still no change. What am I missing here? If i understand the armor skill right it will help with defense and with offsetting the speed reduction of the gnomes, correct? So I really would like to know how to train that skill up to 100 or so. I would especially like this when I start to get steel setup.