Sounds like your knock height and or your arrow rest is off.
easiest way is to take it to a pro-shop and have them tune.
If your a do it yourself kind of guy, take some time and read this manual. It will set you straight on the basics of tuning the bow yourself. Paper tuning is a good start and will get you close, after that I do alittle bare shaft tuning and finish up with my broadheads. If your broadheads are flying with your practise tips then you have a well tuned bow.
There is alot in this manual that doesn't pertain to a compound and alot in there that can make tuning a bow sound like a impossible task so you need to keep to the basics, for you, I would start on the section called paper tuning. That is a very good discription of paper tuning and how and why to make the adjustments, stick to it and it'll have you shooting a straight arrow in no time.
Good luck and don't be shy with any follow up questions or how things turn out.
to 3d shoots,