[Python] cv2 findChessboardCornersSB not working, detects only some corners

I have pictures of my checkerboard, but findChessboardCornersSB() or findChessboardCorners() cannot detect all corners, only some of them, or none, regardless of checkerboard dimensions that I input.

Here is the image of the sample

I usually work with fits file, but for this I exported it to png

img = cv2.imread(filename, cv2.IMREAD_GRAYSCALE)

# Try finding the chessboard
chessboard_size = (6, 7) # Also tried with (7,6), (6,9), (9,6)

flags = cv2.CALIB_CB_EXHAUSTIVE + cv2.CALIB_CB_ACCURACY

flags_std = cv2.CALIB_CB_FAST_CHECK + cv2.CALIB_CB_ADAPTIVE_THRESH

# flags_std = cv2.CALIB_USE_INTRINSIC_GUESS

ret_sb, corners_sb = cv2.findChessboardCornersSB(img, chessboard_size, flags=flags)

ret_std, corners_std = cv2.findChessboardCorners(img, chessboard_size, flags=cv2.CALIB_CB_FAST_CHECK | cv2.CALIB_CB_ADAPTIVE_THRESH) # Standard method

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