Matroska...or notta

Folks, anyone know why all of a sudden, mkv files, are causing a problem on my WD HD Live?
Symptoms are no remote control features available other than stop and play, when you try to fast forward it says
something like this file does not support trick mode, and if you resume after a restart, it simply sits there in a black screen, and no matter how long it is left for, does not play. If you reboot the device, it will play the film, right up to exactly the same point it originally stopped. And when I say stop, that is exactly what it looks like - that the stop button was pressed!

Usual suspects are covered, firmware up to date, tried to do analysis of offending files but nothing obvious.

As I said, it still plays mkv’s that are older than say last week (I heard that  :->) Anyone any ideas?
Its not firmware and is possibly a new encoding practise or encoder?
Very frustrating to get to middle of a film and then have it bomb.
Thanks for any help.

If it doesn’t support “trick mode” it’s almost certainly a bad encode.