Error when importing Substance archive into Unity 5

Most of the time, .sbsar files import into Unity flawlessly. Occasionally, I'll come across a file which will not import successfully into Unity. All the files I am trying to import are authored by me, using the Mac version of Substance Designer 5. When the archive doesn't import correctly, I get the following error:

Failed to retrieve a single SBASM file in the archive... Did you use the Substance cooker to generate the archive?

I have of course tried the usual things: restarting both programs several times, exporting the file from Substance under different names with different settings to different directories, etc., and nothing has worked. Most of my substances do work, but I have two so far that refuse to. If it is of any importance, the two which I am having problems with have substantially more complicated node graphs generating them. The strangest thing is that while they won't open in Unity, the archives open and generate fine in Substance Designer.