My baby isn’t sitting yet!!

Hi guys! My daughter just turned five months on the first of March. And she isn’t sitting up really yet. She can sit up bent over a little bit she can’t sit up straight at all. I know she’s only five months but the thing is she’s already crawling so why can’t she sit? She’s been crawling since around 4 and a half months. How can I encourage her to sit? All she wants to do is crawl. As soon as I put her sitting down she pops into the crawling position. What can I do?!?!