Internet connection, temporary server issues, the size or the format of the video can be the origin of the problem.
First of all, avoid uploading videos from iPad devices, check this article for more info. Also note that videos cannot exceed 1GB.
If you comply with these requirements and still doesn't work, feel free to email us to email@example.com with the original file of the video and we will have a look into it. If the video is too large to be attached, please send us a Dropbox or Google Drive link.