is sometimes related to heridity.Still you can try by
-having mild yogic exercises to increase appetite,digestion,muscular buolt up.
-increasing caloric value of your food.-take more proteins
, sufficient carbohydrates,with good vitamine&mineral suppliments.
-avoid exessive /strenuous work.
-sleep sufficiently(At least 8 hrs in night)
-Syrup Neogadin/Syrup Periactin
2 spoons twice a day taken before meals will help.