Baby cannot sit or roll onto back from tummy - help!

She’s 5 months old in a few days and she still cannot roll onto her back from her tummy. She rolls onto her tummy fine, but just cries when she gets tired and stays rigid when I try to rock her and roll her back onto her back. She cannot sit. I think she may be a little behind, no? I’ve tried to do tummy time a lot with her. What do I do? I’ve tried to use a towel/tummy time support when doing tummy time with her. Just recently, she has laid herself down with her neck/head turned to the right, whining and looking sad