OP, we need more info before we can help you determine the problem.
As previously mentioned, we need to know what type of ammo you were using, (brand, bullet weight) and distance from target. Those are the two big ones. Additional info that would help a further diagnosis would be: indoor or outdoor range? How do you grip the pistol when you shoot (pics?) and if you were shooting offhand or off a rest, like sandbags. All those would help us give you a better diagnosis.