If it is not lining up sideways - lean the top of your head over to see through it, if it is too low or too high reposition it again. Be sure it is tied in well and will not move at all after you have secured it. Put a mark on the side of the peep and the string with a white or yellow marker. If it does move you will see the marks are not lined up. Look down the arrow and find any spot on the rest or the riser where the arrow might be hitting and readjust to eliminate any such collision.
Some older bows have no riser clearance and arrow contact with the riser is virtually unavoidable.
The spot where the arrow is not pointing to the left or to the right is known as the centershot area. Some owners manuals will give an exact measurement for centershot. This will only be a rough estimation. If a difference is visible, the cam timing is probably way off and needs to be reset by a pro with the proper equipment and procedures. Make sure the cable guard is tight and that it is pulling the cables over far enough that the arrow fletchings will not hit the cables at all.