Visitors can't see swf

My site:

It seems as if some visitors can’t see the swf-file load for some reason.

the index.php includes a script that directs users to either start.php (for users who have flash player installed), or to flash.html which is an instruction page on how to install flash player (for users who don’t have flash installed).

The thing is that some people get directed to start.php but still can’t see the swf, and I have no idea what the reason might be

The users seem to have flash player installed