I have to ask, why are you posting your question on here? If your aircraft has failed its rate-of-climb, I woud guess that an experienced engineer is the one who has advised you of this. If this is the case, then he will have used the Flight Manual for your specific aircraft, incuding any performance write-downs supplied by the CAA in the form of Change Sheets and Supplements.
If you have flown the airtest yourself, and can't get the figures right, have a chat with your engineer. After all, he is the one who has to sign it as acceptabe to submit to the CAA.
P.S. when was the last time the aircraft was weighed?
camobe