There are many reasons of why Whatsapp is not sending videos, you may have a poor connectivity, or you don’t connect to the internet, so you can check your connectivity, another reason is the size of the video, as Whatsapp doesn’t send videos with size more than 16MB, in this case you can send it as a file, as you can send a video up to 100MB in size, also if you turn off Whatsapp during sending the video, the sending might be paused, you need to keep it open to continue sending the video, also it may be a problem with the app itself, if it’s down you won’t be able to send your video, and the last choice you might need to update your app.