My 8G Clip, fw 2.01.32, will show but not play 44.1 kHz 16 bit mono FLACs (compression level 5). The same as MP3 works fine.
Workaround: Convert to stereo with identical channels. Then it plays fine, at the same file size as before (as FLAC will notice that there’s no difference between the channels).
Still, it’s a bug that should be fixed.