If you're an ectomorph don't cut, in other words, don't go into a caloric deficit if you want to gain muscle and strength. Don't eat crap, bad fats and sugars, and fix your posture. I suffer from the same thing. When I flex my abs like ripped and big, but when I let go they go out. Check Athlean X on YouTube. They have several suggestion and tips for fixing this problem. Good luck!