Work experience certicate in horticulture for newzealand student visa?

i applied for newzealand visa and i submitted my work experience certicate as working in my grandmothers farm doing cultivation and agricultural management the certicate is signed by vao ( village administrative officer ) but they say its not enough with that i also submitted the land doents details and letter from my grandmother as i am looking after her land , still the officer not convinced what else i should show for agricultural work experience? i am confused i asked the visa officer in phone the answer she said is 'you should know what doents to submit its not my work to tell what doents to submit and she cut the phone so if any 1 know any details about agricultural work experience certificate pls help