This could be because it is a Podium Front end and App Backend.

To fix this navigate to:
  1. onMessage
  2. Website
  3. Podium Site
  4. Click into the Page Name
  5. Select Manage Downloads Category
  6. Click Organize, and edit the Sort Order of the Items

Or

Navigate to:
  1. onMessage
  2. Website
  3. Content
  4. Downloads
  5. Find the Downloads Category
  6. Click into the Downloads Category
  7. Click Organize, and drag and drop the Files in the order you want them to Display