i have put two skill points into spell warding (increases your spell resistance for each piece of light armour equipped) yet my current bonus remains at 0... and yes I'm wearing a light armour piece.
Edit: it works when i equip another piece or similar armour (another head piece) but not for the piece i was wearing when i purchased the skill upgrade... which sucks considering its an armourset piece and i now need to make another one(it was upgraded)