How do I fix this stupid jump animation not transitioning

I have a jump animation, which plays in up frame if the player is moving up, and a down frame if the player is moving down. this works fine. I transition into the jump animation by setting a bool “Jump” to true, and am trying to transition out by setting it to false. for some reason this doesn’t work. Here’s the code:

        if (groundCheck != null)
        {
            IsGrounded = true;
            animator.SetBool("Jump", false);
        }
        else if (groundCheck == null)
        {
            IsGrounded = false;
            animator.SetBool("Jump", true);
        }

that’s it. I have no clue why this doesn’t work. I even made the IsGrounded bool public to check on it, and it works fine, detecting when I’m in air and detecting when i’m on the ground. PLEASE HELP

@falconstrike209 Does your animator have a transition to another animation/state for the bool Jump,false? It could be helpful to watch your animator run during the game to see if the transition is happening to fast or not at all.