can't change all pixel in image Mat opencv

I have a gray image and I want to change all pixel value in that image into white colors.

I try these code:

int main(int argc, char** argv){

cv::Mat img = cv::imread("C:/mata.jpg");

for (int i = 0; i < img.rows; i++)
{
    for (int j = 0; j < img.cols; j++)
    {
        img.at<uchar>(i, j) = 255;
    }
}
imshow("result", img); }

size of image: 37 x 29

I've checked the total of pixel in that loop, and the result, I can access all pixel but not all pixel in image changes into white color (just a quarter of image changes into white colors, the others still have the same color as original image).

Anyone can help me?