Poor healing. What is causing this? Diabetes?

Could be. Diabetea can be the culprit of poor healing specially if the blood sugar is poorly controlled. Also vitamin deficiencies can be the culprit. www.drlugo.com.
Possibly. Poorly controlled diabetes can cause poor wound healing. Other possible causes: cancer, cushing's syndrome, poor circulation, malnutrition, increasing age, infection or ineffective treatment, poor wound packing/protection...
Yes but many others. Poor healing can be due to diet deficiencies (protein, vitamin), diabetes, drugs (corticosteroids, coumadin, (warfarin) chemotherapy) inappropriate treatment (wet to dry, dakin's, continued pressure) bacterial load or biofilm, osteomyelitis, age, disease (infection elsewhere, heart or arterial, metabolic) autoimmune (lupus, psoriasis, pyoderma, .