7 month old not sitting

I have a 7 month old but she's not sitting yet, she just falls over. I've already started weaning her and she sort of sits in the chair for a short while (it's a bumbo style seat for the moment until we get the highchair) but then slumps forward or to the side.

Should I be worried about her not sitting yet? I've seen other babies younger than her be able to move into a sitting position and sit independently so I am feeling a bit worried. I don't know whether it's something I should go to the doctor about just yet or not.

She was born bang on 37 weeks because her growth charts had really slowed down but there was never any explanation for why this happened. So I don't know if this has anything to do with the slow development.

I don't really know what to do, would appreciate any advice or guidance from your good selves please.

Thank you!