Since you haven't told us the type of generator it is or how it is being used we could only make wild guesses as to the problem and that's not productive. We need more info.
Please elaborate your problem.
Have you checked Its rpm? have you checked its maintenance hours? have you checked its voltage manually with a voltmeter on its output MCCB or breaker?
Please ease us by doing these tests.
If you have checked these problems, then look into you generator control relay how much voltage it shows on it display?