My little one is 6 months old. She loves to roll over, does mini press ups when on her tummy, likes to stand and bear most of her weight when I am holding her and can sit up well with a little support. However if I sit her up and let go (plenty of cushioning around mind) she will only stay upright for moments. Is this normal? I thought at 6 months she would be able to sit unaided. Should I be concerned?