Hypoallergenic milk suggestion and review please

My baby is 2 months old s26 Ha milk nya pero palasuka sya and she did gain much weight. Underweight po sya. Nagswitch po kmi ng similac tummicare hindi na xa palasuka and she gain weight but my problem is hard poop xa. Parang constipated c baby umiiyak c baby kapag mag poop. Any suggestion what to do? Do i need to switch milk? Ask ko po about nan hw and enfamil gentlease thank you po