Certain instruments not working

[Xx_theawesomeblackblock_xX]

I noticed that sometimes, certain instruments will be completely muted (emit no sound) even though most other instruments do work and the settings for that instrument do not mute it. For example, the bassline for a song I'm making sounds weak because the Clean Guitar, one of the instruments used for the bassline, does not emit any sound. Never-used instruments still work properly via loading.