That is a common issue with deep wounds. It could be a couple of things. The first is that the wound is healing from the outside in. This causes a pocket which is perfect for microorganisms to breed and infection to set in..or their may be a little bit of debris still inside that is foreign material and the wound may heal around it but the body knows it doesn't belong so the dog's immune system kicks in in an attempt to remove the foreign debris...either way...the wound needs to be reopened and the dead tissue needs to be removed. If it is really deep a wet to dry dressing needs to be applied...basically a ball of gauze drenched in normal saline and the outer gauze being dry. With this, the saline will keep the inner part of the wound moist so the gauze doesn't stick (if it did and you removed it, it would rip the new tissue away that is starting to grow). With the gauze coming into contact with the tissue...it will stimulate tissue growth from the inside out which is what you want...and a pocket wont form...that and some antibiotics and the pooch will be good as new. I never had to do this with a dog...but I do it with people and it does the trick...if you do this...make sure you keep the gauze covered so the dog can't pull at it...maybe an ace bandage over...and if you can...get one of the cone collars that go on animals to keep them from biting themselves. Good luck