Video downloads are limited to 10 per day.
Once you click to download a video, you'll see the "The download started..." message under the video.
You are able to download the same file as many times as you need (if it fails) but you have to remain on that page, without reloading it. If you reload the page, the download counter will update and count as one download.
You can also visit the "Downloads" page/tab in your browser and re-start the download from there.