As you said, it may be high body fat. You can go to the gym to do an individual fitness test first (mostly free) to see if your body fat is normal or high. It is not easy to show muscles when body fat is high. It is recommended to reduce fat first. If your body fat is normal, it means that your exercise method is wrong. You can exercise your abdominal muscles mainly by rolling your abdomen. You can do sit-ups, sit and lift your legs, roll your abdomen sideways, roll your abdomen and turn around.
If you roll your abdomen with weight, you can use dumbbells or discus, put the weight on your chest and roll your abdomen. If you want to be more difficult, you can straighten your hand.
Hanging legs is also good. If you want to carry a load, you can lift dumbbells with your feet and lift your legs with your stomach.
There will be more ways to go to the gym, such as: belly rolling machine belly rolling, tension machine belly rolling.
Hope to adopt!