Why is my butt implant wound still open on day 21? (Photo)

Unfortunately the central single incision for buttock augmentation with implants is notorious for wound healing complications such as yours. Typically this will not heal without removing the sutures and debriding any dead tissue and applying dressing changes several times a day for the next month or two. If when doing this implant becomes visible through the wound (i.e. exposed) then the implant will likely have to come out and be left out for at least 5 to 6 months before it can be replaced. Glad to be of help.

In my experience, wound closure issues can happen if the implant is too large.  Keeping the implant size proportionate to your body is a great way to heighten the success rate of the operation.  Be sure to choose a board certified plastic surgeon that specializes in this procedure and is well known in the medical community for it.  

You may have an infection with communication to the implants. Often times the best solution is to remove the implants if this is the case. Best of luck.