What is the cause for failure for wound healing?

Many things could cause it. Here are a few reasons:

Inadequate nutrition (wounds need protein and vitamin C to heal)

Diabetes that is not properly controlled

Infection

Trauma to the wound

Lack of good blood flow to the wound (some chronic diseases, such as atherosclerosis or venous insufficiency, can reduce blood flow to an area)

The amount of rest the client is receiving (an exhausted body is not good at healing wounds)