Eventually I solved it: (based on Word 2016 for Mac) I had the same problem: the numbering skipped two numbers, jumping from 6 to 9.
I tried everything, but neither one of the solutions described above worked for me.
So this is how I circumvented the problem: I assigned the two footnotes after the missing ones special characters (7 and 8).
Thus they were not counted as part of the continuous series.
Restore download from Science Direct in Find Full Text 3. Improvements to CWYW when sharing libraries with ambiguous citations 5.