hi dean.
one possibility is that you haven’t adjusted your FCP user prefs. (under the FCP menu)
he default number of audio tracks is 8, which is ludicrous.
up them to 24 at least.
filters use up track locations, so thy may have pushed you over the edge there.
another possibility is that you are using mp3 or AAC audio.
dont.
it may work, but it may not, and you cant tell when it wont, it’s unpredictable,
convert all audio for FCP to AIFF or WAV
cheers,
nick