I know it's been a long time since you asked this, but I'm curious if you ever found an answer.
Just tonight I was working in Flash on my 64bit Mac Pro running Snow Leopard (10.6).
I was happily "onion skinning" when all of a sudden they wouldn't show up any more. I read your post and tried changing my settings from Action Script 3.0 to 2.0 and sure enough, that fixed it.
Is this just a bug? No fix still?
I haven't had that problem for a while, although I'm not sure if it was related to the file or if one of the software updates fixed it. If you want to attach your file I'd be glad to take a look and see if I get the same problem on my machine.