Option to jump to Previous Slide has disappeared

I just finished building a course in SL3 and created custom navigation buttons (Next, Previous, etc.).  On all slides, the "back" button trigger is set to "Jump to slide"...."previous slide."  This has worked for the two weeks that I was developing the course.  No issues.  I published the course and have loaded it onto our LMS.

This morning I went into the SL project to tweak a couple minor things, and I discovered that the option for "previous slide" no longer appears in the options.  There's only "next slide" and then all the slides in the course.  It doesn't work on a brand new slide, and if I go to edit an existing trigger that's set to "previous slide," even though it currently is set to previous slide, when I select the options, previous is not there.

Any ideas??