Hello, I would be happy to assist you.
Since your abdomen was opened with one big incision to perform this operation, it is possible that your muscles were not reattached. If so, when you use your abdominal muscles they would contract on the sides and you'll have a bulge in the center. If you do not notice this, then it may be simply that the exercises you are doing are not to the capacity that would actually cause you to build muscle. This is very common. It would be best to see a physical therapist to help strengthen your muscles. I recommend you start with seeing your doctor to make sure you do not have a hernia that needs to be operated on. If not, you can be referred to physical therapy to help strengthen your abs. The exercises would be much more intense in order for you to gain muscle mass and strength.
I wish you the best. If you have additional questions, please ask. If none, please rate my answer. Thank you.