Version/build of BS.Player?
Did you already try "Filter management"?
(rightclick >) Options > Preferences > Filter management
Further: preferably access filters/codecs directly.
With a file loaded you can see what filter/codec BS.Player actually uses for playback:
(rightclick >) Options > Filters > Advanced (also open the [+]-es)
(You can also adjust filter's/codec's settings there.)
What is shown with an flv-file loaded?