What affects the trajectory of an arrow?

The force with which it is fired and the angle at which it is fired.

The horizontal component (which I assume you are trying to increase) of the trajectory can be made bigger by keeping the arrow in the air for longer by either firing with a greater force or by aiming slightly upwards (not too far or it will start to land closer to you).

If you want further explanation I'd be happy to help you out. Just email me at jamesmclennan@hotmail.co.uk