Sometimes an experienced ewe can sense when something is amiss with a lamb, such as a heart condition. It makes sense for her not to waste her energies pointlessly. Twinning on to another ewe who has just given birth might work (dip the lamb into the birth fluids of the new ewe and present it with the newborn for her to wash.) A wise older ewe may stick to her guns about rejecting one lamb, so bottle feeding if you have time is an acceptable option.