Flash components are easily played online, because most Internet browsers are equipped with special plugins.
On the other hand, FLV files stored on your computer need special codecs.
The latest versions of this codec come with some useful changes, such as bogus tags being ignored when opening a file and seeking tags, repairing HEVC streams
files during playback, fixing parsing of metadata strings, corrected Speex audio codec work, fixed seeking and playback etc.
This lightweight splitter may be exactly that one thing you are missing in order to play FLV files successfully.