Charactercontroller.isGrounded never triggers

Hi, I'm just trying to implement basic jump ability on my 3D character, then realised, perhaps since beginning game, priestController.isGrounded is never triggering at all (I confirmed with print console command). Using Unity 2018.1.

This is still all in the Update() function at moment, not FixedUpdate(), but I assume that's not the issue. The movement and rotation of character all work fine, and they fall off things and fall back down to ground via gravity OK. It is standard CharacterController component, I have no rigidbody component on the character, do have Animator component on character in case relevant.

Radius of characterController is 0.7, Skin width of character controller I changed from 0.08 to 0.01, still isGrounded not triggering.

Other answers mention things about the order of the Move function related to checking .isGrounded, something about moving to not grounded state each frame, but not sure if and how this could help in my case. A thousand thanks for any suggestions.

 if (!hasRotationGravityFrozen)
             {
                 // is grounded is never triggered at all
                 if (priestController.isGrounded)
                 {
                     print("I'm grounded!");
                     gVelocity = Vector3.zero;
                     if (canJump && Input.GetKeyDown(KeyCode.Space))
                     {
                         gVelocity.y = 0.5f;
                         priestController.Move(gVelocity * Time.deltaTime);
                     }
                 }
                 else
                 {
                     // this is working fine
                     gVelocity.y += Gravity * Time.deltaTime;
                     priestController.Move(gVelocity * Time.deltaTime);
                 }
                 // this is working fine
                 if (this.transform.position.y <= -30 && !gravityKilled) {
                     gravityKilled = true;
                     Gravity *= 3;
                     GravityDeathSequence(); }
 
                 // and this is working fine
                 currentRotation.x += Input.GetAxis("Mouse X") * sensitivity;
                 currentRotation.y -= Input.GetAxis("Mouse Y") * sensitivity;
                 currentRotation.x = Mathf.Repeat(currentRotation.x, 360);
                 currentRotation.y = Mathf.Clamp(currentRotation.y, -maxYAngle, maxYAngle);
                 Camera.main.transform.rotation = Quaternion.Euler(currentRotation.y, currentRotation.x, 0);
             }
 
             if (!hasMoveFrozen)
             {
                 // and this is working fine
                 Vector3 forward = Camera.main.transform.forward;
                 Vector3 right = Camera.main.transform.right;
 
                 // The .normalized here scales the Vector to 1 unit size
                 Vector3 forwardDir = new Vector3(forward.x, 0, forward.z).normalized;
                 Vector3 rightDir = new Vector3(right.x, 0, right.z).normalized;
 
                 Vector3 move = (forwardDir * Input.GetAxis("Vertical")) + (rightDir * Input.GetAxis("Horizontal"));
                 priestController.Move(move * Time.deltaTime * Speed);
             }