My daughters six months old and not sitting up by herself yet. I know they say they should be able to by 8 months but I feel like everyone baby sits up by 6 months! She is a bigger baby almost 20lbs so I’m not sure it the weight is harder for her to support herself or what. She’s already crawling and hit every other milestone early. Should I be concerned? She’s can like half way kinda sit slanted but not upright.