Refresh token not working after access token expiration

While creating a zoom meeting through my php application, the procedure I followed are :

  1. I fetched the access and refresh token through database table(zoom_settings).

  2. After the access token expiration after 1 hr (it is working fine for 1 hr), I called a function to update access token and if success, it will also update the database table with new access and refresh token.

  3. Inside that function , I have called a post method to /oauth/token with authorization (clientKey : secretKey) in header and in form_params, I have passed a refresh token with grant_type => refresh_token. it is shown in the attachment below.

I am getting an error in this function while updating access token. I have followed the exact same procedure that is mentioned in the zoom official documentation.

The error in my error log says :
Client error: POST [] resulted in a 400 Bad Request response:
{“reason”:“Invalid Token!”,“error”:“invalid_grant”}

Please have a look in this issue, as I am getting this error for quite a long time now.