Inconsistency in Rigidbody.MovePosition

Hi, I believe I have gone extremely wrong somewhere or I have found some sort of bug. Probably the former. Basically, I have two Rigidbody.MovePosition instances that both do different things when called.

 function FixedUpdate () {
     rb.MovePosition(transform.position + transform.forward * -thrust * Time.deltaTime);
     print (-thrust * Time.deltaTime);
 if (Input.GetAxis("Accelerate") != 0)
     {
     rb.MovePosition(transform.position + transform.forward * -thrust * Time.deltaTime);
     }
 }

(This is a simplified version of my script but I am sure the rest has nothing to do with this except for setting the rb variable as a rigidbody).

When I press the accelerate button, the rigidbody moves. But, if I am not pressing the button it does not move. FixedUpdate should be called every physics update but it is not moving the object. When I use print, it returns "-0.6" yet doesn't move.

Any and all help is appreciated.