Why am I losing some inputs on Update function?

I wrote a simple C# script to control my character using CharacterController component, but I'm losing some Inputs from "Jump" button and I don't know what is happening.

My logic is very simple: I'm reading the input and calculating the next movement to be done in Update function and using CharacterController.Move on FixedUpdate function with the movement calculated before.

Here is my code:

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof(CharacterController))]
 public class SimpleCharacterController : MonoBehaviour {
 
     //-------------------------- ATTRIBUTES --------------------------
 
     public float MoveSpeed = 4.0f;
     public float GravityForce = 0.3f;
     public float JumpStrength = 0.1f;
     public string JumpButton = "Jump";
     
     private float verticalSpeed = 0.0f;
     private Vector3 finalMovement = Vector3.zero;
     private UnityEngine.CharacterController controller;
 
     //------------------------- AWAKE FUNCTION -----------------------------
     
     public virtual void Awake()
     {
         controller = GetComponent<UnityEngine.CharacterController>();
     }
 
     //------------------------- UPDATE FUNCTION ----------------------------
     
     public void Update()
     {
         // Calculate character movement horizontally(x, z = movement) and vertically(y = gravity/jump)
         this.finalMovement = calculeMovement () + calculeGravityJump ();
     }
 
     private Vector3 calculeMovement()
     {
         float horizontal = Input.GetAxis("Horizontal");
         float vertical = Input.GetAxis("Vertical");
         
         return
                 transform.forward * vertical * MoveSpeed * Time.deltaTime +
                 transform.right * horizontal * MoveSpeed * Time.deltaTime;
         
     }
 
     // This method saves the current vertical speed to calculate the movement in the next updates
     private Vector3 calculeGravityJump()
     {
         // If the character is on the ground, vertical movement is nullified
         if (controller.isGrounded)
         {
             verticalSpeed = 0.0f;
         }
         // If character isn't on the ground, apply gravity to the vertical movement
         else
         {
             verticalSpeed = Mathf.Max(
                                       - GravityForce, // Limits maximum fall speed
                                       verticalSpeed - GravityForce * Time.deltaTime // Apply gravity to current speed
                                       );
         }
         
         // If press to jump and is able to jump, add vertical force
         if (Input.GetButtonDown(JumpButton) && isAbleToJump())
         {
             verticalSpeed = JumpStrength;
         }
         
         return Vector3.up * verticalSpeed;
     }
 
     public bool isAbleToJump()
     {
         return controller.isGrounded; // A character is able to jump whenever it is on ground
     }
     
     //-------------------------- FIXED UPDATE -----------------------------
 
     public void FixedUpdate ()
     {
         this.controller.Move (this.finalMovement);
     }
 
 }

Thank you for the help.