it is hard to give an advice... :-)
Normally I have to take a look at how you shoot.
There are some "principles":
1. If you have the right technology, you can say, hitting left or right is a problem with your technology. If you´re hitting up an down you don`t practise enough because you don`t have the "eye" for the distance
2. If you don`t have an established technic there a lot of mistakes you can make. The most I`ve seen are problems with the release. Aiming, twisting the bow, arrows to long, to hard or weak, walking in the drawlenght, Fletching too high or too short etc are also probs.
The arrow is a high complex construct so there is not only one answer
Greetz
Cord