Subsequent Webcam Frames Are Identical

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I'm coming across the problem where subtracting two subsequent webcam frames gives a completely black image - I would have thought that it would leave a non blank image when there is movement in the video stream, but I always get a blank image. I think the problem could be that the two frames are identical, but I think this is highly unlikely because the camera is (at a guess, v4l2 doesn't support FPS property) at 20 fps tops. Here is the code I'm using:

#include <iostream>
#include <opencv2/opencv.hpp>

int main() {
    cv::VideoCapture input(0);
    cv::namedWindow("last_frame", cv::WINDOW_AUTOSIZE);
    cv::namedWindow("current_frame", cv::WINDOW_AUTOSIZE);
    cv::namedWindow("difference", cv::WINDOW_AUTOSIZE);
    for(;;) {
        cv::Mat last_frame;
        input >> last_frame;
        cv::Mat current_frame;
        input >> current_frame;
        cv::Mat difference; 
        cv::subtract(current_frame, last_frame, difference);
        cv::imshow("last_frame", last_frame);
        cv::imshow("current_frame", current_frame);
        cv::imshow("difference", difference);
        if(cv::waitKey(10) >= 0) break;
    }
    return 0;
}