If you select the track (the buttons to the left and right of the screen) is the LED green or red? It could be muted. Press Function + Track button. If it is red while selected then it is unmuted, green is muted, blank is muted with the track not selected.
Also if you are not in recording mode (Rec button is not solid red) then hold function + Up arrow to select track mutes. Then the sequencer trigs will be lit based on muted and unmuted tracks. The first 8 are the sample tracks and the second 8 are MIDI tracks. LED on = not muted.