I had this problem when the server I was uploading to did not support the 
mime type


> Have you tried making a preloader?  In other words, something (created 
in 
> a separate scene) which will play while the movie is loading?
> 
> Otherwise, have you tried asking Flash to simulate loading the movie 
over 
> various connections?
> 
> To do this, assuming you're on a PC, do [Ctrl]+[Enter] from the work 
> area.  Once the movie is running inside Flash, choose your speed from 
> the "Debug" dropdown menu.  Then do View > Show Streaming.  This will 
> simulate loading the movie over the desired connection.
> 
> Your movie probably takes a while over 56k modem when it loads almost 
> instantly on your machine.
> 
> Try streamlining things like movie clips, and removing stuff that is no 
> longer being used, like extra frames, buttons, library items, etc.