DC MOTOR ROTATION problem with ARDUINO !! Answered

hey there ,i have a simple problem in my controlling motor with arduino project that i send 5v to it at the top of the code but many times it gives a very small incomplete rotation (a small step i mean ) and stops the code not allowing to complete it , if i give a hand help to rotate it rotates properly and some times without help . so how to make it rotate without hand help ? and what is the idea ? thanks .(i am using l293d)