RaycastHit2D ground detection

Hi, Im making a 2d platformer game where the player can jump once and if he kills something midair he can jump again. Now im using RaycastHit2D to check if the player is grounded but now I have the problem that somehow the player can jump twice without killing anything midair.

Im trying to solving this problem for a long time now and have no idea how to fix it. Does anyone have a idea how to fix this?

here is the part where I check the ground:

     private void CheckGround() {
         // Mid Air Control
         RaycastHit2D groundRay =  Physics2D.Raycast ( this._transform.position, -Vector2.up, 1F, 1 << LayerMask.NameToLayer("Ground") );
         // Grounded
         if ( groundRay ){
             isMidAir = false;
             anim.SetBool("isMidAir", false );
             jumpEnabled=true;
         }
         // Not grounded
         else{
             isMidAir = true;
             anim.SetBool("isMidAir", true );
         }
     }