Have you done a fecal to rule out parasites as reason for no weight gain? Keeping her and a buddy in a smaller area would be ideal. Let them bond deeply before setting them loose with the herd. Most times they will stick together, eat together and sleep together. I have sisters here that if one gets off too far the other cries for her. A bell can help the blind one hear where the other is at but what you hope for is the buddy watches out for the blind one.