You need to make sure you are using the dragon souls to unlock the words of shouts as you pick them up. This took me 2 weeks to figure out (and I was unemployed when this game came out so it was all I was doing). When you go into the shouts screen, you should see it asking you to unlock a shout by pressing a certain button. This will use a dragon soul, and also give you one more word during that shout. After all 3 words are gathered, there is no more way to power them up.