Two things to check (refer to the owner's manual section on external controllers, page 18 in my version): is the FC4a plugged into the "sustain" jack and is the menu setting correct - Utility/select "03:controller". The FC4a is a switch that is either on or off and you need to have the proper setting (not "half damper" which is for the FC3a, a variable controller like a volume control). If you get the setting wrong sustain won't work properly. (I don't have my MX here to walk through the settings-left it with a friend before the pandemic-and I'm using info from the manual). hope you can get it worked out.