Does anyone know why some files on Share can't be added to the downloads? About 5% of the files I come across can't be downloaded even if I use the "Add to Download" option.
These are duplicate files (AFAIK, with different uploader IDs). Since you may already have added one of these to your download list, Share will not let you to add the same file again ;)
Check the hashes to be sure.
This is another litte Share quirk...