How can I find the problem of my M8200 Kubota farm tractor for not cranking?

With key in starting position, it just clicks.With boost from car it might turn engine over twice really slow and then click. Like there is a lot of juice going to ground. Or could it be a relay? Battery cable terminals checked. Battery ground cable checked. For about 2 or 3 months, would have trouble getting engine to crank over. By turning ignition key back and forth to the starting position, after some time, it would start. Now it just clicks with tractor battery alone.