Unfortunately, IIRC, "support" does not work like we are used to understand it.
When you order a unit, or for that matter a corps to "support" another unit or corps, in fact they are going to act as a "backup", meaning they will try to carry the primary unit's order's if that unit is unable to do it. So, if you order a inf unit to "support" an arty unit in fact what they will do is, for example, carry an attack that the arty was doing, if the arty is forced to retreat or is wiped out. Not very logical, IMHO...
In fact, the real "support" (ie, defense of arty units by inf or cav units) is carried out by the AI or by micro-management. It helps to station units near by the arty, maybe the posture of the units also has some influence ( I am waiting for a more detailed explanation of what the combination of defensive posture and high initiative can really do for arty), maybe the corps commander attributes, can also help. But many times you have to micro-manage, which is a bit unfortunate (although the foot arty AI is much better now in defending itself than it used to)
I do not know if JMM is planning on improving this part of the orders or not.
Regards