The prompt line offered DIFFERENT FILES. Nothing I
could see of a comparison, no third window opened. So, the frame did
something but didn't let me know the result.
"Different files" means that there weren't enough points of similarity
between the files or DF blocks to compile a meaningful list of
differences. To some extent this is driven by quirks in funcs FD
(FindDifference) and FM (FindMatch), which aren't amenable to
tweaking. Try DeFining larger or smaller sections of the two files.