If you are using Safari for Mac, the web browser itself has a bug when uploading that Apple hasn't fixed in years. I used a workaround for that bug, but javascript has to be enabled and I'm not 100% sure it works 100% of the time as I have had some Safari users have trouble still. I don't know why this is as I can't reproduce it on my end in any version of Safari.
Also, sometimes uploads get corrupted in the process (especially on a slow or unstable internet connection), and just fail. So you just do it again. Pretty soon, the site will connect directly to 23andMe, so users will not have to upload anything. It will be as simple as clicking a button. This should make it so easy, even your grandma can do it (no offense to grandma's).