You may feed other brands like S26, Similac, Nan Pro or other brand you know of in the market. Buy in small tin to try. You can also try to give a little bit of breast milk then formula milk. Mix feeding in between, so your child get the difference taste of milk. Perhaps maybe due to formula taste, stomach upset, too much of milk, can be any reason. Try on different tips however if still the same, seek advice from doctor/nurse during monthly check up.