Apparently this is fairly common, because we usually spend so much time with bub during the day a bit of 'tension' can build up, especially if mum and bub are both tired. When someone else takes over the settling bit then bub can feel the difference and relaxes and goes to sleep. I was really worried about DH going back to work after Henry was born as he always managed to settle him, and after a few awful days I managed to settle him by myself......deep breaths to keep calm! You will be fine, try not to worry (easier said than done, I know).