The buttock is first on the list of places pressure sores form, and for a wound there, the single most likely answer is that the pressure has not stopped.
The rule is that pressure sores tend to form where skin covers bony areas with less tissue under the skin to pad the area. Buttocks come first, ahead of the elbow, hips, heels, ankles, shoulders, back and the back of the head. The cause is simple: too much pressure on the skin for too long reduces blood flow to the area. Without enough blood to nourish it, the skin can die and a sore may form.
A wound in this spot is loaded every time someone sits or lies on their back, which for many people at risk is most of the day.
So pressure relief is the treatment, not an extra. Change positions often: every 15 minutes in a wheelchair, about every 2 hours in bed. Use pillows, foam cushions, booties or mattress pads to reduce the pressure, chosen with a provider based on the wound and whether the person is in bed or in a chair. Do not use donut-shaped or ring-shaped cushions, which reduce blood flow to the area and may cause sores. Avoid slipping or sliding when moving, and avoid positions that put pressure on the sore.
Moisture is the other factor specific to this site. Not being able to control your bladder or bowels makes a pressure sore more likely. Use a moisture barrier to protect the area from bodily fluids if needed, and ask your provider what type to use.
Who should be treating it depends on the stage. Stage I or II sores will often heal if cared for carefully. Stage III and IV sores are harder to treat and may take a long time, and most stage III and IV sores will be treated by your provider.
And the wider list of reasons wounds fail: infection, diabetes, poor circulation, nerve damage, bone infection, being inactive or immobile, a weak immune system, poor nutrition, excess alcohol use, smoking, a foreign body. A wound that has not started to heal in 2 weeks, or not completely healed in 6, is called non-healing — and pressure sores are first among the types treated at a wound care center.