RZT 50 won't turn over, but starter engages

I have an RZT 50 mower (model # 17AI2ACP056) that will not start. When I turn the key the starter clicks, but does not turn over. I've replace the starter, starter solenoid, spark plugs, oil change, air filter, fuel filter, and it has a brand new fully charged battery.

Any ideas on what I should check next?