should i be worried?

ariana dah 8 months++...but everytime i see her, i feel like she's still in her second or third month...until now dia cuma meniarap jer...ke depan ke belakang pun belum...she also cannot sit on her own...tapi kalau i put her in a babychair kat restaurant, bolehla duduk coz she rely on her bam bam body laying on the chair...other babies at her age sudah boleh panjat2 dah...should i be worried?other than that she's perfectly healthy...i know that different babies with different development stages but still i cant help feel a bit concern about it...haiz...