Generator clicks but wont start

Clicking like that generally indicates low voltage at the starter motor. As others have suggested, that can be due to poor connections somewhere along the way, either on the ground or the positive side. It's possible it may be a poor connection inside the generator particularly if the starter solenoid is separate from the starter motor (in which case it could be the wire between the two).

If the batteries are old, it is also possible that their voltage is sagging excessively under load. The generator starter is a pretty high current load, around 90-100A for my (4KW) generator.