There are many different faults that can cause your symptoms.
Do you think the starter is engaging or is the click you hear just the relay?
It would be good to measure the battery Voltage to see what it sucks down to, that's the best indication of battery health, if that doesn't drop below 10 it should be cranking, less than that the motor might be stalling.
Make sure your battery ground is well connected, clean both ends and apply some Vaseline. I cannot stress enough the importance of Vaseline on the battery terminals.
If all you hear is the relay click it's the dreaded Startus Interuptus caused by too much resistance (0.2 Ohms is too much) feeding the Start relay resulting in low Voltage to the solenoid.
A little homework you can do is measure the trigger signal, that's the wire from the Start relay to the starter solenoid it should measure 11 Volts at least when you push Start.
The trouble is you need to measure it when the starter fails. I would wire a 12 Volt lamp to the trigger terminal of the solenoid and just tuck somewhere you can see at a glance next time it fails to crank the brightness will indicate the Voltage close enough.
BTW the tail light is an indication of Voltage at start, it should only dim a little while cranking
Something else to do is learn how to hot wire the solenoid, its just a matter of a jumper from battery positive to the trigger terminal (Must be careful to put the bike in Neutral). Some guys wire in a heavy duty button to do that but I would just use a wire with a spade connector from the solenoid ending near the battery.
The Startus Interuptus fix on the Jackal is a little more involved because it also feeds the headlight relay.
I'll send you a PM if you want to go that route.
