My 6 month old won't sit up.

My six month old daughter will not sit up. She's crazy about tummy time and striving with everything she has to crawl but she gets so cranky about being immobile when placed in the sitting position. What should I do? I've taken a lot of advice from her occupational therapist but none of it seems to work. (having her sit between my legs with a toy to entertain her, putting her in her boppy, etc.) I thought maybe her exersaucer or walker might help but she is still way too small for these toys. (She was a month preemie and had gastroschisis at birth, on special formula, doesn't gain weight easily, so she is a little short, about 27 inches and a little over 12 pounds.)
I'm reaching out to you and your experience, fellow moms. Please help!