I'm not certain how to get this to work, but it sounds like one patch (instrument) is playing all of the notes that it's receiving from Band-in-a-Box. When you press Play in Band-in-a-Box, BB is sending the synth patch changes on different MIDI channels. For example, it might specify a bass patch on channel 2 and a piano patch on channel 3. This information probably isn't being seen by FL Studio.

After that, and while the song's playing BB is sending all notes that need to be played by a bass patch on channel 2, and all notes that need to be played by the piano on channel 3. The plugin is probably playing all notes with piano or some other single instrument.

Maybe there is a way within FL Studio that you can assign certain instruments to certain channels before you press Play in BB. You might want to mute the BB drum track until you have sorted out how to get everything else to play.

You can see which channels BB is using for each track, by going to Opt. - Preferences - MIDI channels.

Andrew
PG Music Inc.