Some background. A common use for SMA will be that the instructor wants to reveal answers and solutions to the SMA version. Therefore the SMA version shouldn't be exactly the same version as the for-credit version.
So when WW tries to randomly generate the new SMA version, it compares the HTML from the original to the new randomone it just made. If they are exactly the same, it tosses the new one out and tries again. I think 3 to 5 times, according to some config variable.
If it never finds a truly new version, you get this message. Usually that means the problem isn't capable of making new random versions.