What causes skin ulcers that won't heal?

Lots of things.: Ulcers on legs can be due to infection, poor blood supply, or even skin cancer. Ulcers on buttocks are often due to pressure from prolonged sitting or bedrest. Ulcers in the mouth have many possible causes. Chronic infections can cause nonhealing ulcers just about anywhere on the body.

Many things: The skin is a complex organ. When skin does not heal it suggests something is missing in the pathways that the skin takes from break down to being healed. Most commonly seen are high glucose from poorly treated diabetes, low blood proteins in malnourished patients, and low oxygen in patietns with perpheral artery disease, excessive bacterial colonization leading to infection and skin cancer.

Various reasons: There are a number of important reasons why wounds fail to heal, but the top of my list is poor treatment such as using peroxide, betadine, drying it out and so on. Then consider diabetes, and poor nutrition. Failure to eliminate the base cause especially pressure, or swelling (as in venous leg ulcers) and poor blood supply, lastly in a long standing wound you must consider skin cancer.