Shooters can be very fickle ... You didn't provide enough info for me to get my hands on to help -- such as whether the head is homemade or store-bought, length of head, line weight, leader, type of running line, rod weight, and casting stroke.
You might want to begin finding a fix by reviewing my series on the shooters posted on the Forum under Fly Fishing Articles -- Everything Else.
If you are doing a homemade shooter, you also might want to read All About Lines, Part IV from Fly Fishing for the Rest of Us. http://www.activeangler.com/articles...air/lines4.asp
Problems most fly fishers encounter in throwing homemade shooters usually relate to (1) overhang - the distance outside the rod to the rear of the head, (2) the overall length of the head, (3) the leader butt diameter and leader length, and (4) the casting stroke.
On the other hand, don't forget that a lot of folks would give their eye teeth to consistently throw a curve. Don't forget what you are doing and lose the technique.
One other point -- it's quite possible that you may be twisting your wrist in the micro-second before stopping the rod on the forward cast. Remembering the line will go where the rod tip stops, a curve is the likely result from that snap of the wrist; in fact, my positive overhead curve cast requires just this setup.
Hope this helps. Come back with any questions.
PS. Airflo's Multi-Head system is very good...