Maybe you should have a look inside your .sub file
Or at least its size:
if it's a few Ko big and there's text inside, maybe it's not a bsplayer-supported format (some formats example at the end of
this page). I believe sami and ssa formats are not supported.
If so, you should find a converter and use SubRip or MicroDVD format
(there are plenty of tools on
www.divx-digest.com or the previous linked site)
if it's rather close to 1 to 3 Mo (or more...) and nothing clearly readable inside, I'd say it's a vobsub subtitle file, which means you also need a .idx file with it (which contains the timestamps) and bsplayer doesn't display those either, you need
vobsub (at least directVobSub)