Taknak minum susu..

hi mommies.. my baby 4mnths old.. sy bagi formula milk.. past few weeks dia taknak minum susu btl2.. even dah 6 7 jam pun still taknak minum.. klau i paksa bagi minum dia minum 1 2oz je. ataupun tidur hisap teat botol. knpa ye? pls help