I've had this happen with my users from time to time. There are a couple reasons that can cause this:
- Large file size
- Poor Internet Connection
- The fonts included cannot be rendered via Proof
- A glitch in the matrix
Usually if I'm able to download the file myself, I'll test and see if I can make a Proof. If I can than it's usually the user's internet or Workfront had a quick tantrum. Either way, I ask them to try deleting the version and trying again. Once did I come across something strange where the Proof rendered but still said generating so I enlisted the help of Support to fix it.
If I can't create a Proof I open the file to see if I get any error messages or anything and then submit a Support request if necessary.
Hope this helps!