Sounds to me like your amp isn’t getting signal sent to the sub channel. Take a known working RCA from one of the other known good working channels such as a full range or midbass (preferably) channel nearby and plug that RCA into the sub channel. Play the music again and if the sub plays than you have no sub signal from your HU or possibly bad RCA. If the sub channel still doesn’t play, well than you may have a bad amplifier sub channel or blown amplifier fuse.