I find this works when I amend a return I did not do originally. (as a side note I charge a full return fee not my amended fee as you are actually doing 2 returns) I do the original return and save the file. Then I make a copy and save it with usually original behind the file name. (this also helps if you really mess up the amended and want to start over when it just won't populate the original column on the 1040x). 

Then in the first file make sure select efile (fed and state if needed) and the box that says original return was efiled. 

That usually works fine then. If you have the original efiled box checked then maybe you didn't select efile?