The first time we run, we try to convert the TTF to WOFF, SVG and EOT for use on HTML5. If it fails, we print a warning. After the first time, we do not warn anymore.
Doing a -clean build forces the tools to process the TTF file again, in case the failure was due to something silly 
Not all fonts convert properly, and the tool we use is no longer in active development. Not sure if there’s a better command-line font converter that’s cross-platform and doesn’t require FontForge or some other big dependency