my bb also has the same problem. We went to health checkup when she was 6 mths old. The nurse said my bb has not got enough strength on the leg and also not keen to grasp a toy with her hand. Need to have another follow up review by 9 months old.
In the meantime, I try to give her more practice of sitting and standing. When sitting, try to encourage her to sit by herself, not letting her to lean back so much.
Different babies have different development speed, we should not worry too much but need to give bb plenty of chance to practice so that they can master the skills law.....
Is your baby thin? Mine is very thin, maybe that's why cannot stand long or sit well
The nurse did not ask me to change milk powder, when I told her that she drinks about 25.5oz per day, she said that the volume of intake is ok, but need to practise more with bb on sitting, standing and holding toys..... she said that I should give bb some rice congee with meat.
I used to give bb Friso but now changing to A+ No 2 as I think she grows fatter with this one. When she was new born, I gave her breastmilk with 1 meal of A+ and she was gaining weight ok but then when I got back to work and change all to A+, she didn't liked it and therefore we changed to Friso. She drank ok with it but just didn't gain as much weight as before...... So now decided to change back to A+ No2 la.
My 99 also gave her mashed potato mixed with A+ and some mashed carrot, bb loved it, maybe you can try this one also.