New Raspberry Pi with OSCM - problems

Just installed OCSM on my Pi 2.

I seem to be able to play avi videos but not mp4s. Have read around a bit and it seems well established that some file types won’t play, however other people don’t seem to have a problem with MP4s. What might be going wrong in my case? When I try to play them nothing happens - sometimes I get “…”

Any advice would be very welcome!