vinccilau,
i think all baby are different don't worry lah, as long as your bb is healthy n happy, no need to push or rush them to do things that they are not ready mah... but maybe u can try play with your bb in bed with some toys (u lie next to your b attract him / her? with the toy, just a little practice for his / her muscle is good for them anyway)
best wishes n good luck! x