There are two things that can cause this. One of them is the @ symbol in the file or folder name. This causes big problems with creating torrents because of how it affect the bencoding of the .torrent file. The second possibility is you may have exceeded the number of files that you can handle in a single torrent. This is most common with picture sets. If you have picture sets, it may help to roll only the pictures of the set into a single ZIP file but make sure to NOT put the videos themselves into the ZIP file.