There's one CODEC combination which always fails for me in RasPlex when using 'Direct Play' setting, but works fine when I force transcoding on by selecting a bitrate setting instead of "Direct Play". Apparently that enforces transcoding for everything, which in itself is a bug (it should only enforce transcoding for material with higher bitrate). But for me this bug is fortunate, as I would otherwise be forced to play these media files encoded to some crappy bitrate, just to make transcoding kick in.
Anyway, this 'Direct Play' failure occurs for my collection of 'Farscape (1999)' which uses CODECs as shown below (customized "MediaInfo" list):
-- File --: Farscape s01e01 Premiere.avi Video #0 : 50mn 20s 684ms : 776 Kbps DivX 3 Low : 512x384 (1.333) @ 23.976 fps Audio #0 : 50mn 20s 664ms : 96.0 Kbps CBR 'MPEG-1 Audio layer 3' in 2 ch @ 48.0 KHz
I'm pretty sure that it's the "DivX 3 Low" video CODEC that causes the problem, though I don't understand exactly why.
All normal DivX and XviD media work just fine, and on my PCs I've never had any problem with this show.
I suspect that the DivX/XviD CODEC implemented for the RPi is less generic, and doesn't include compatibility to old variants like 'DivX 3 Low'.
Best regards: dlanor