My pet is a 14 year old retriever. Somehow he developed a wound on his underside that we got stitched up. But stitches did not stay and the doctor stitched him up again after 20 days. Now the stitches are again loosening up and the gap emerging. Why is his stitches not healing? He has a small tumour on his underside next to the wound. The doc doesnt want to touch as my pet is too old.