Morty Brew

Hi I received a Coopers home brew kit for Fathers Day and enjoyed the process of brewing this with my son, we followed the instructions and video to the letter, our beer tastes ok although with a slight aftertaste, sort of molasses however this is not my issue When I pour the beer I manage to get a head with a bit of aggressive pouring but the head quickly disappears and the beer is quite flat I checked and cleaned my glasses well (bi carb mix) but this has not helped Does any one have any clues to revive this brew