Foxy Lady:

Did you check to see if the stems or branches on the sick side got crushed or damaged?  Petunias are so delicate that just a little pressure can damage the stems and they don't recover.  Maybe you need to cut it back a little more on that side.

If I could see it I might have a better idea.  
As Tiger said, at least it's perking up on one side already.  That's a start!