drag and drop and multi file selection is only expected to work in modern html 5 web browsers like IE 10 or 11 should be ok, older browsers are going to have to select and upload 1 file at a time.

if you have trouble using a modern web browser then you need to use the web browser dev tools and look for any script errors in the console and also look for any script or css  files that fail to load in the network tab. Make sure for example that you have applied any needed skin changes if you upgraded from an older version. any kind of script error on the page even if unrelated to the file uploaded can prevent the uploader script from working.

Other than that, everything I know about troubleshooting file upload problems is in the article Supporting Large File Uploads.

You can also try our demo site and see if things are working differently there vs your site.

Hi Graham,

I've updated the troubleshooting section in the article Supporting Large File Uploads, to mention that drag and drop and multi file selection is only supported in modern web browsers.

Note also that if most of your users are stuck with legacy XP machines, it is possible to hide the dropzone to avoid confusion but that will mean the drag and drop won't work for anyone. But if you want to do that just add this to the theme.skin file of the skin:

<portal:jQueryFileUpload runat="server"
    UseDropZone="false"
/>

I definitely think using Chrome on those machines is a better option though.

Best,

Joe