So i keep seeing loads of posts of babies half crawling or sat up fully unsupported. My baby girl is 6 months in 3 days and she still cannot sit up straight without leaning on a sofa or something. Nor can she crawl. She hates tummy time no matter what i do to entertain her whilst shes doing it, she will try to move slightly then when she realises she cant, she starts grunting and then crying until i roll her back over to her back. She also cannot roll from back to front or front to back yet. She has started rolling more to the sides though recently. Should i be worried?