1998 Dodge Caravan Serpentine Belt

We had a new engine put in the van, shortly after on a very rainy day it threw the serpentine belt, the shop failed to put the shield back on under the car, I thought putting this back on would fix the problem. It didn't every time it rains hard or snows it throws the belt off. I have had to put the belt back on at least 20 times. The belt does not appear to have any kinks or twists in it. What causes this or what should I look for to fix this? Is this a recall or waranty issue. I know that it is a safety issue because I lose my power steering while driving.