Hi there,
there can be several reasons for that - as i am a Flash Designer i can tell
you only of those "typical" ones independently from our Maemo system.

First of all, it could be that the ppl. at Joost scripted the whole thing so that
only the latest Flash plugin is allowed. By talking of "latest" we are talking about
so many sub-versions (e.g. 9.5.3.2.1) that this realy sucks.
If they did so, you will not be able to view the page.

The other thing could be that your Flash version is not the latest one.
Try to visit a website like mine (www.mojocafe.net) on your device and
if you see a Flash animation than everything should be fine.

Hope to be of help to you!

Carlos