Why do I still have a lump after umbilical hernia surgery? (photo)

I had an umbilical hernia surgery 4 months ago. I had it done open incision with stiches NOT mesh. Despite losing weight, fixing my Diastasis Recti and doing exercise I still have an evident jump out where my belly button is. It looks like the hernia is still there! I had another ultrasound to check the hernia hadn’t reappeared but it hadn’t. There is some scar tissue there. Why is there still a lump where the hernia was? If it’s scar tissue how can I flatten it?