Hemoglobin drop after Dialysis

Has anyone had issues with a drop in hemoglobin numbers after dialysis? My sister has been receiving two units of blood on a regular basis following her 3 weekly sessions of dialysis. She has had two colonoscopies and two procedures looking for possible leakage in the digestive tract. No definitive results. This is a very depressing situation for her. Can anyone offer any information regarding this condition?