Recently I was having a problem where a Flash video was not being displayed. It turned out the Flash video had not been created properly and had a dependency on other Flash files that were not available. When I was viewing this Flash video directly using JBoss I did not see any errors. But when I viewed it using Apache the logs showed the dependency problems.
Did you try watching the HTTP traffic with a tool like Fiddler or Charles?
If there is a dependency on something and it failed to download it then you should be able to see the 404 or some other error.