My DD went through a similar thing with day sleeps when she was about two months old - she would only sleep in our arms or lying on my chest. Nights were all OK. It turned out that it was because I was eating foods early in the day that gave her bad colicky pains - like Sultana Bran for breakfast, and snacking on loads of nuts. I started giving her Brauers Colic Relief, and then cut out dried fruit and nuts, and now she sleeps like a champion, and I no longer need the Brauers. I was told by a MCHN that lying on their back can exacerbate any digestive pains, which is why they will quite happily sleep upright. Obviously the food thing might not be the case for your bub, but just thought it could be something to think about? It really made an immediate difference to my DD as soon as I identified the problem, and I didn't need to do a whole elimination diet or anything.