I don’t usually work on IIS machines and there were too many unknowns for me to narrow down the issue very much but some quick research combined with my experience with Linux servers led me to find these points to research further:
IIS won’t compress assets if it is computationally too expensive (an article suggested it would need to be using less than 80% CPU to honor compression requests).
Using and compressing fonts have lots of weird quirks like: (on Linux servers) the files themselves have to be set to ‘executable’; only some font formats can be compressed; Open TrueType fonts seem to have 2 different MIME types (font/otf and application/x-font-otf) so double check that either/both is configured.
In Apache and Nginx, an admin typically has to restart or reload the server before configuration changes take effect, does IIS have the same requirement and if so, did you do so?