There has been some rumbling in the forums about drag and drop failing. This seems to be related to a recent update by Firefox to version 18. I have not seen it in person yet, but If you would like to install an older version (17) you can get that here for PC or here for Mac. This issue has also been seen in Chrome.
Moodle.org is aware of the problem and is working to fix the issue. You can see the Tracker post here. You can also see a couple discussions describing the issue:
I am keeping a close eye on this and will update as I can. Please contact me if you have any questions.