Hello, My son is 20 weeks old. Hez born at 36 weeks with birth weight 2.1kgs. his current weight is 6 kgs. He is trying to move towards his sides but unable to roll over. When I'm putting him on his tummy , he feels uncomfortable. But hez able to lift his head when put on tummy. He reached his other milestones like recognising us, making sounds, trying to hold objects, but unable to roll over. Is is normal or do I need to be worried. Thanks a lot 🙏🏻