Force in AddForce with ForceMode.Impulse insconsistent

Sometimes my player jumps relatively high, but then he jumps relatively low. I tried several methods, but none of them worked.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine.SceneManagement;
 using UnityEngine;
 
 public class PlayerController : MonoBehaviour
 {
     public Rigidbody rb;
     public GameObject cam1;
     public Transform playerBody;
     RaycastHit hit;
     RaycastHit hitGround;
 
     public float MoveForce = 10f;
     public float jumppadForce = 10f;
 
     public bool hasKey;
     public bool isJumping;
 
     private void Update()
     {
         if(Physics.Raycast(cam1.transform.position, cam1.transform.forward, out hit, 4))
         {
             if (Input.GetMouseButtonDown(0))
             {
                 switch (hit.transform.gameObject.tag)
                 {
                     case "key":
                         Destroy(hit.transform.gameObject);
                         hasKey = true;
                         break;
                     case "lock":
                         if (hasKey)
                         {
                             SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex + 1);
                         }
                         break;
 
                 }
             }
 
             if (Input.GetMouseButton(0))
             {
                 switch (hit.transform.gameObject.tag)
                 {
                     case "movableJumppad":
                         hit.transform.SetParent(playerBody);
                         break;
                 }
             }
 
             if (Input.GetMouseButtonUp(0))
             {
                 switch (hit.transform.gameObject.tag)
                 {
                     case "movableJumppad":
                         hit.transform.SetParent(null);
                         break;
                 }
             }
         }
 
         // part of jumping mechanic
         if(Physics.Raycast(transform.position, Vector3.down, out hitGround, 1.2f))
         {
             switch (hitGround.transform.gameObject.tag)
             {
                 case "jumppad":
                     isJumping = false;
                     break;
                 case "movableJumppad":
                     isJumping = false;
                     break;
             }
         }
         else
         {
             isJumping = true;
         }
     }
 
     // Update is called once per frame
     void FixedUpdate()
     {
         // jumping mechanic
         if (!isJumping)
         {
             rb.AddForce(jumppadForce * Vector3.up * Time.fixedDeltaTime, ForceMode.Impulse);
         }
 
         transform.Translate(Input.GetAxis("Vertical") * Time.deltaTime * MoveForce * Vector3.forward);
         transform.Translate(Input.GetAxis("Horizontal") * Time.deltaTime * MoveForce * Vector3.right);
     }
 }