I have a "new to me" Vision recumbent that has 9 rear gears and a SLM 750 thumb shifter. The rear shifter will only move the derailleur through 4 clicks/5 gears. I have loosened and tightened the cable. I can cause the derailleur to use a different 5 gears, but still only clicks 4 times. I know the shifter is supposed to go through 9 gears. Any suggestions?