They can just click because of several reasons like: Low power such as low or bad battery, loose cable connection(s) or solenoid coil connections. It can also be a bad or seized starter motor....it can also be that the engine can't turn over as in locked-up. I would check all the cable and wire connections then pull the clutch cover and see if the primary clutch rotates in both directions. If so, then look to the starter motor.