5x5 is a good system, but you may need to tweak it just a little. As the others have said, you could experiment with higher rep ranges. Or you may want to try some slightly different variations on it. I've posted an article about that recently:
http://www.strongandfit.net/2011/03/5x5-workouts.html
You also need to be sure you are deloading every once in a while. About every 6-8 weeks you should be cutting back on the weight and volume. Cut back to about 60% of the weight you normally use. This will seem very easy, but that's the point--to give your body a rest.