6m old cant sleep on her back in the day

Hi mummies/daddies. My second child (6m) cant sleep on her cot during the day. She needs to be held to sleep, and will wake up every 15-30mins. And hence, she only take 1 nap during the day, lasting about 2 hours. This does not happen to my elder son; who can easier sleep on the bed himself after a few pats on his butt. And takes 2 naps during the day when he was an infant. This makes me realised, all babies are different 😮‍💨 Is there any tips on letting her sleep herself? I cant do anything the whole day thanks to her.