Jumping effectively in a 2D sidescroller

So, I´m developing a pretty simple 2D sidescroller.

The player basically has two actions: switching through layers in the scenery and jumping obstacles.

Regarding the switching layers component, it´s all set and working. On what it comes to the jumping function, tough, I´m facing some troubles regarding "delay" in the response.

As you can see in my PlayerJump script below, I´m using FixedUpdate() and a Jump() function to take care of the jumping action:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerJump : MonoBehaviour {
 
     private Rigidbody2D player;
     private PlayerResponse resp;
     public float speed;
     public float jumpUntil;
 
     public float impulse;
 
     public Transform playerY;
     private float yAxis; // y position when player is not in the air
 
     // Use this for initialization
     void Start ()
     {
         player = GetComponent<Rigidbody2D>();
         resp = GetComponent<PlayerResponse>();
         playerY = GetComponent<Transform>();
 
         //Gets the coordinate of the player in the Y axis 
         yAxis = playerY.transform.position.y;
     }
     
     // Update is called once per frame
     void Update () {
 
     }
 
     void FixedUpdate()
     {
         Jump();
 
         if (playerY.transform.position.y <= yAxis)
         {
             playerY.position.Set(playerY.position.x, yAxis, 0);
             player.constraints = RigidbodyConstraints2D.FreezePositionY;
         }
 
         Jump();
 
     }
 
     void Jump()
     {
         if(playerY.transform.position.y > yAxis)
         {
             return;
         }
         if(resp.pressedOnce()) 
         {
             Debug.Log("JUMPED");
             player.constraints = RigidbodyConstraints2D.None;
 
             player.AddForce(Vector2.up * impulse);
             
             return;
         }
     }
 }
 

The "jumping" on itself is working: the player presses the space bar, and jumps. The thing is, a lot of times, when pressing the space bar, the jump doesn´t effectively work for some reason, and is essentially ignored, while other times it isn´t.

I´ve heard this ignorance might be due to using FixedUpdate() and something about it not being as constantly called as Update() is. But I´ve also heard you can´t do physics stuff on Update().

So how should I proceed here? How can I make sure that every time the player presses the space bar, the jump happens without exception?