it's normal and it happens. One finger is typically more dominant. For example, I almost always lead with my middle, whereas most people lead with their index.
Also remember that the index finger will be striking the string in a different area than the middle will. You can't change the spacing of the fingers in your hand, so that'll be there forever.
Try playing lines only with your index to build it up. There's a great exercise that Gary Willis does to help players get a consistant attack from both fingers...here it is around 3 minutes:
http://www.youtube.com/watch?v=md8FCeqNrcM