If you are attempting to upload media from your iPhoto or iMovie, it may fail to upload. This is caused by the way iMovie handles clips when they are being transferred to your computer. If you right click on your file and go to info, the file size is very different from what is being uploaded. You'll notice the same behavior regardless of where you are uploading it (whether it be WeVideo, or Dropbox, or Google Drive, etc.).
So any clips directly transferred from your device to your computer has to go through one more step before it can be used on the web. It has to be exported or saved into a different format (MPEG-4 preferably). You can do this usually by going to File --> Export or File --> Save as within iphoto and imovie.
Alternatively, you can use WeVideo for iOS to upload directly to your account.