Embedded Flash in PDF not working

Hi all, hope someone can help with this. I've published a Flash 6 movie and
embedded it in a PDF using Acrobat Pro 7. Works fine for me, but when I send it
out to certain people they get the following message:

Cannot play media clip because there is no player available that can play the
media while satisfying the author's intentions as well as your security settings

The users in question are running XP, have flash player 6, and acrobat reader
7 - which all should be capable of handling this. Is there a setting I need to
tweak from within Flash?

Any suggestions? I really hope there's a solution to this as I've spent a fair
bit of time working on this only to be thwarted at the last!

Thanks

- B