Why can’t i get a flat stomach and chest ?

I’m a 16 years old boy and between march and july i lost just over 2 stone and have maintained a weight since then of about 10’7 – 10’12 stone! The problem is over the last 3 weeks or so ive been going to the gym 3 times a week doing a mix of cardio and weights for hour sessions, and making sure i eat 3 healthy meals a day, and i just cant get a flatter stomach or chest. the rest of my body is slim like arms and legs, but my stomach and chest are really pudgy. What should i do to help get towards a 6 pack stomach rather than a pregnancy stomach lol!