Hello Darren
I am not using Maplex for this task because I plan on publishing out as map service.
I also ensured the placement was correctly set for all label classes.
As a test I tried changing attribute that I was labeling on and all 16 label classes have the proper placement. Although I don't understand why?
Since, I could not figure out the proper solution as a workaround I changed the placement of the 2 label class to below the line which is closest I could get to being on the line.