My little girl is almost 4 months old (beginning of May) and I feel she’s a little behind? She loves her belly! And when you pick her up she will hold her arms out pushing off your shoulders. But, when she is doing tummy time she cannot lift herself up. She just lays there face flat into the floor or on one side. Is there anything I can do to help her get stronger? She has a big ass head lol so I know it’s heavy. Also because of her head, she’s not so great at sitting up. She’ll keep her head up and mostly balanced by herself, but if we were to let go she has not the core strength needed.