What Causes Poor Appetite And Poor Weight Gain In A Child?

hi gudevening. I have a daughter who has completed 3 years of age on 1oth of april 2015 and has entered 4 years. her weight is 13 kg. She is a very fussy eater and avoid eating heavy food including breakfast, fruits and vegetables etc.... I have visited child specialists who has advised her to take beevon vitamin but is of no avail. Yesterday she had taken NOWORM medicine. I am tensed as her health is deteriorating gradually and she is becoming too lean and thin which had affected her concentration on studies. Do please advise.