On the psp's web browser?
As psphome said, you must enable flash from the psp's settings menu (all the way to the left, down to 'system settings' then down to 'enable flash player')
Sony ported flash 6 to the psp, its quite possible the content you're tying to view requires features of flash 7 or 8.
In short, blame sony
The whole web browser is next to useless IMO.
From sony's update info:
# [Internet Browser] now supports Macromedia Flash contents playback.
# You need to enable the Flash contents playback in the [System Settings].
# The version of the flash player is Macromedia Flash Player 6 (a part of the functions is not supported).
So not even flash 6 is fully supported, most sites use 8 these days.