Change instrument not adding a new midi channel

I have a score in which a guitar on a single staff changes from acoustic to electric (both in tab), but the sound patch defaults to clean electric and I'd like to change it to overdrive. I've been told that I can do this by utilizing the new mixer channel created for the electric guitar by the change instrument feature, but it didn't do this. All that's shown is voice, piano, acoustic guitar, and drumset in the mixer, preventing me from changing the new sound patch. Is this a bug or can I fix it? Thanks!