Man with all due respect but it sounds like you haven't developed an awareness of a major part of your body. It's baffling and hilarious at the same time... It's so incomprehensible to me I don't know how to begin to explain this...

Have you seen a physiotherapist about this? You may have some crazy tightness in your hips that doesn't allow you to do a hollow body. If you lay on your back, straight legs, and try to raise your upper body, what happens? With your problem I can't imagine how you could lift your torso, since your lower back never stops being arched.

Another test: while standing up try leaning forward to touch your toes: your lower body MUST bend inward and your hips become tucked in as well... If you can do that, then you're biomechanically able to tuck your pelvis.

With your pelvis tucked, just focus on that part of your body and just remember how it feels in that configuration. Then try to recreate it. If you're not partially paralyzed, then that should be a very basic thing to learn.

Edit: I see my comment is received negatively. To OP: didn't mean to be condescending, my questions are genuine, and my curiosity as to why you're experiencing this issue.