Getting black or white image when calling mss screenshot function second time

Question

I used mss module (https://pypi.org/project/mss/) to make screenshots. When calling function second time, I get black or white image. The weirdest thing is that ocasionally it succeeds.

I tried giving some time like sleep(1) between calls but it didn't help.

import mss
import mss.tools
from PIL import Image
import win32gui

def screen_part(top, left, width, height):
    '''
    Returns a PIL image of the part of the screen
    '''
    with mss.mss() as sct:
        # The screen part to capture
        monitor = {"top": top, "left": left, "width": width, "height": height}
        output = "sct-{top}x{left}_{width}x{height}.png".format(**monitor)

        # Grab the data
        sct_img = sct.grab(monitor)

        # Convert to PIL Image
        img = Image.frombytes("RGB", sct_img.size, sct_img.bgra, "raw", "BGRX")

        return img

def window_coordinates(win_name):
    '''
    Returns rect for the window --> tuple

    '''
    x = y = w = h = None
    def callback(hwnd, extra):
        if win32gui.GetWindowText(hwnd) == win_name:
            nonlocal x, y, w, h
            rect = win32gui.GetWindowRect(hwnd)
            x_ = rect[0]
            y_ = rect[1]
            w_ = rect[2] - x
            h_ = rect[3] - y

    win32gui.EnumWindows(callback, None)
    return x, y, w, h

def capture_left_figure(win_name):
    '''
    x_karty / (x_table + w_table) * (x_t + w_t)

    '''

   x, y, w, h = window_coordinates(win_name)

    left = round(439 / 1084 * w) + x
    top = round(1743 / 1963  * h) + y
    w = round(50 / 1084 * w)
    h = round(39 / 1963 * h)

    print(left, top, w, h)

    img = screen_part(top, left, w, h)
    return img

img_1 = f.capture_left_figure('window_name1')
img_1.show()

img_2 = f.capture_left_figure('window_name2')
img_2.show()

First image is always fine. Second image crushes most of the time, sometimes (especially when window is big) it surprisingly works well. I have no idea what might be the reason. I'll be greatful for any advice.

EDIT: I exchange mss screenshot function to pyautogui and it's still the same. It looks like the problem is somewhere else. This is new function:

def screen_part_2(top, left, width, height):
    '''
    Returns a PIL image of the part of the screen
    '''
    img = pyautogui.screenshot(region=(left, top, width, height))

    return img

EDIT2: Coordinates for the second image are fine. Even such a code gives a mistake when calling function second time (also checked by printing coordinates before taking screenshot):

img_1 = f.capture_left_figure('window_name1')
img_1.show()

img_1 = f.capture_left_figure('window_name1')
img_1.show()