I had the same old problem of that the missile wasnt moving once i fired it. after i checked it, i couldnt tell what was wrong.
if you look back to all the responses from people who have had the problem, they all have completely different answers. I suggest you just delete everythnig you have done (including the movie clips) and start this section of the tutorial all over again. by doing this you are sure to make it work. but what i did (because i am lazy) is i just went to step one, and downloaded their finished example. because i i know that if i did it again it would work, and some problem that isnt humanly possible to pick up will be fixed.
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