I can't Jump on imported objects,My GroundCheck is not working

I was following a Bracky’s Tutorial about the basic movement in unity (FIRST PERSON MOVEMENT in Unity - FPS Controller - YouTube) when i’m not on an imported object i just jump just fine but then when I came to realize that when I jump on Imported Objects (such as models imported form blender) I all of a sudden lose the ability to jump then when I hope back down to the ground i fall down very quickly. is there any fix to this?

Heres the code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerMovementScript : MonoBehaviour
    public CharacterController controller;

    public float speed = 12f;
    public float gravity = -9.81f;
    public float jumpHeight = 3f;
    public Transform groundCheck;
    public float groundDistance = 0.4f;
    public LayerMask groundMask;

    Vector3 velocity;
    bool isGrounded;
    // Start is called before the first frame update

    // Update is called once per frame
    void Update()
        isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance , groundMask);

        if(isGrounded && velocity.y < 0){

            velocity.y = -2f;

        float x = Input.GetAxis("Horizontal");
        float z = Input.GetAxis("Vertical");

    Vector3 move = transform.right * x + transform.forward * z;

    controller.Move(move * speed * Time.deltaTime);

    if(Input.GetButtonDown("Jump") && isGrounded){
        velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);

    velocity.y += gravity * Time.deltaTime;

    controller.Move(velocity * Time.deltaTime);

i have the same issue. is there a solution?
,i have the same issue, have you found a solution?