Advanced Prop Spawner Problem

I was trying to make a missile launcher out of the AMRAAM from PHX and the AMRAAM box. I attached a thruster to the back of the missile and used the advanced prop spawner so I could repeatedly spawn functioning missiles in the launcher box. If I spawn a missile on the ground and activate the thruster, it will not move. If I freeze the prop spawner in the air and spawn the missiles in the air, they do work. As the missiles do not function when spawned on the ground, they do not shoot out of the launcher box when activated. Does anyone know how to fix this?