Are you using new subtitle engine as suggested by
Ico-man?
(rightclick >) Options > Preferences > Subtitles [Subtitle properties]
Then check the box "Use new subtitle parser/renderer"
Also there: is "Draw subtitles to Overlay surface" selected. What if you deselect it (or vice versa)?
What is selected as "Rendering mode" under
(rightclick >) Oprions > Preferences > Video > [Video rendering]?
What version/build of BS.Player are you using?