A more thorough cleaning of the wound to remove infected tissue.
As I say this is not an area in which I have current expertise so may not apply to your problem at all. I do recall from my training seeing pilonidal sinuses (which are probably not dissimilar to what you have) that these things can take a long time to heal and may need prolonged intervention. Is it in an awkward spot that is prone to rubbing etc?
How do the staff who are treating it say it's doing?