help!!!

my almost 6th month old (on the 30th) can't sit up on her own. She hates it when I put her in that position and either starts to scream or instantly goes to her belly. (she is obsessed with being on her tummy.) I am getting so concerned because I feel like most 6th month old either sit up already or they almost can. has anyone else experienced this or have advice? How is she ever going to learn?