Has anyone else seen the LEARN assessment tools stop working?

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We have created instances of pre and post test and mapped them to a course, which is then mapped into an annual learning plan.  We have been experiencing, sporadically but regularly, a situation in which the user has one of the following occur:

1.) They open the course, the pre-test doesn't open, but the course allows them to complete the work (should not be allowed according to the settings we've chosen in LEARN).

2.) They open the course, the pretest and course work goes fine, but then the post-test won't work.

When I investigate this further, many of the users originally mapped to a pre/post test have be not been assigned that instance.  Again, this is sporadic, without any consistent pattern as to user type or corporate location. The only way to fix this is to report out all of the current pre/post comparisons, or pretest scores only in one course, then reset the assessment (wiping out all of the data on scoring to date).  We've had to do this several times now, and will have to compile the scoring comparison data by hand in order to validate and report on