Failure to respond to stimulating agent can be due to iron deficiency, nutritional deficiency (e.g. Vitamin B), inflammation or infection. Make sure the patient is receiving the correct weight based dose when starting an ESA. When has been stable and there is a suboptimal response to ESA it is important to assess possible causes and treat accordingly.
Kidney Disease: Improving Global Outcomes (KDIGO) Anemia Work Group. KDIGO Clinical Practice Guideline for Anemia in Chronic Kidney Disease. Kidney Inter., Suppl. 2012; 2: 279–335.