Anyones baby NOT sitting unsupported at 8 months old?

My daughter will be 8 months old next week and she just will not sit up on her own unsupported. Idk how to get her to do it. My oldest was sitting up unsupported at 4.5 months old so I never dealt with this last time but she will ONLY sit if she’s being supported by someone/something. The second the support is gone, she throws herself backwards and won’t even try to sit. She can roll over, pull herself up on things, she stands in her walker and jumps up and down in her jumperoo, she sits up perfectly fine in a high chair and her sit me up seat, she sits fine on my lap, she can even army crawl around the floor she just won’t sit up by herself. I try to practice with her every day by letting her sit with a boppy pillow behind her supporting her, I have a little head support back pack thing that I put on her so she can practice without hitting her head when she throws herself backwards, she’s just not interested and legit doesn’t even try. Is she behind ? She has hit every other milestone so far for her age besides this one.