Hi,
There are various reasons why a wound does not heal properly. Infection of the wound is a common reason for poor wound healing. Sometimes, infection by atypical bacteria can also cause poor wound healing.
Other causes include recurrent trauma, poor vascular circulation and trapped foreign body in the wound. An underlying skin cancer can also present with a non healing wound, though this is much less likely in your age group. Diabetics also generally have poorer wound healing.
I would suggest seeing your doctor or dermatologist for a proper assessment and treatment of the wound.