(Suppose you're referring to 1.37.826.)
I'm afraid .ssa subtitles are only partially implemented in BSPlayer's native capacity to display subtitles. So to get similar results like you do with WMP, you should (also) make use of VobSub (as supposedly used for WMP) in BSPlayer. To this purpose:
(rightclick >) Options > Preferences > Filtermanagement > [General]:
check/select both: "Allow intermediate filters" and "Enable advanced graph building"
and under ...Preferences > Subtitles > [VobSub]:
check/select "Enable VobSub"
Please let us know the result.
Ou en bref:
Préferences > Gestion des Filtres > [General] ...
? cocher: "Autoriser les filtres intermédiares" et "Activer la gestion avancée"
et
Préferences > Sous-titres > [VobSub] ...
? cocher: "Activer les functions VobSub"
P.S.:
If no change: could you let us know which codecs/filters BSPlayer uses for playback: this is shown by BSPlayer (with the video file in question loaded!) under (Rightclick >) "Options" > "Filters" > (Advanced).