The big difference in shooting between PES 6 and bascially every other PES before and after is that it punishes and discourages shooting from any angle or position.
If you are used to PES 5, where you can just shoot full power on goal no matter where you stand, what your angle and distance to the ball is etc., you will have quite the learning curve head of you. But ultimately it is a system that rewars playing proper football and feels far less arcadey than any other football simulation.
Most of these things are intuitive and hard to explain, but you will get a feeling for when to shoot. The more "control" you have over a ball in a situation, the closer you aim towards where you player and the ball are looking/moving etc., the more accurate and less sensitive your shots will be.
But try to just run vertically and shoot in a 90°, try to shoot right after gaining the ball, shooting without really having control over the ball, or trying to shoot with enemies near you will mostly lead to the shoot bar instantly filling up and your shot going to the moon.