having trouble using parallel computing in image proccesing

Hello, I'm trying to process a video and create an output video from modified frames.

because I have a multi-core machine I'm using parfor to accelerate the process,

the problem is that every iteration all of the Matlab workers take the same frame instead of reaching for the next one. any advice? my code:

 video = VideoReader('file_name.mov');
outputVideo = VideoWriter(fullfile('destenation folder\','output file name.avi'));
outputVideo.FrameRate = video.FrameRate;
open(outputVideo);
frame=0;
while hasFrame(video)
    if frame>=video.FrameRate*5&&frame<=video.FrameRate*7
        images = cell(1,6);
        parfor frm=1:6
            frame = frame +1;
            img = readFrame(video);             
            images{frm} = RetrieveOutputimg(img);
        end
        for image=1:6
            try
                writeVideo(outputVideo,images{image});
                figure;
                imshow(images{image});
            catch
            end
        end
    else
        frame = frame +1;
        readFrame(video);
    end
end
close(outputVideo);