Is my 6 month old behind?

We are in a baby class ages 4month-10 months and there are 2 other 6 month olds in the class and they can roll from belly to back, sit up, along with other things that my 6month old doesn’t do yet. I know all babies develop differently but I can’t help but get down about it! I’m a mom I’m going to worry. When should I really start to worry about sitting up and rolling over from back to belly? My little one still HATES Timmy times.