Hi, Adrian, Maybe your stance and cue action is not straight. Try to hit two balls (lining straight) in a short distance and hit it slowly and see if you can hit it straight and even to put the cue ball and the object ball into the same pocket. I usually use the middle pocket to practice. You may also try hitting one ball by passing through the spot points, say from brown ball spot points to black ball spot points and let it follow through and see if it will return back to the starting point which is now the brown ball spot point. Remember not to hit the ball with any side (不要落西打). In this way you may know whether you are hitting the ball straight. If not adjust your stance or cue action until improvement is made. This way you will learn and understand your own cue action and stance and see if you are hitting straight.
P.S. Anyone have other suggestion and correct me if I am wrong.