Insufficient data provided to be able to give any conclusive answer.
(
In fact no information given whatsoever.)
Version of BS.Player?
Some data on your PC and OS?
Virus security installed?
BS.Player does play flv:
[FIXED IN 2.33] Can BSPlayer v2.31 play online movies? (FLV)
When you load a flv file what filters/codecs are shown under:
(rightclick >) Options > Filters > Advanced ?
With respect to the delay maybe read:
http://forum.bsplayer.com/general-talk-support/8369-solved-bsplayer-freezes-2-sec.html#post30489 The video freezes for the first 2 seconds Picture Freezes while Audio+subz play
Also your anti-virus program might be the cause.
I assume flv-extension is associated with BS.Player?
(rightclick >) Options > Preferences > General/File types ?