I get the same thing, my abs seem to be the last thing to come in when dieting down. That and lower back.
I have separations in my legs and arms, veins in forearms and sometimes in my legs after cardio or leg workouts yet abs are under a nice thick layer.
I've never tried intermittent fasting, or any other kind of fasting. What works for me is to eat smaller portions more often. I have a huge appetite and its impossible for me to go more than 3 hours without food, yet I seem to have a slow metabolism since the only thing that will get me shredded is keto with low calories.
Are you plateauing? Did you lose a bunch of fat and now stuck on those last 10lbs or so? If that's the case maybe switch up the diet or try eating more/less often, depending on what you did before.