Nature's brutal and if they just keep finding them in the fields it's probably the mother rejecting the offspring based on them being sickly, thus unlikely to make it, and keeping herself alive instead of expending energy for a lost cause in her eyes, instead of being overhandled by the troons . If they didn't even know the mother was pregnant then it's not likely she was getting her nutritional needs met, especially so given they aren't likely meeting the base needs of the animals to begin with.
A well fed, well cared for sheep, with proper husbandry is much less likely to reject her offspring. Proper husbandry being present to know she'd been bred, aware of the pregnancy, checkups to keep her healthy and aware of the pregnancy progression, and having your lambing done in a safe environment when you know it's close to her time.