Re previewing files.

Hi, I am aware of the feature but when I tried to use it on these "films" I got a pop-up window saying there was a problem viewing the files. I get this a fair bit. What happens I thought, maybe wrongly, is that the file needed codecs that were not available in the preview.
I have downloaded several films with this pop-up and no preview available but when I click to play the full thing, (after virus scans etc) using VLC Media player, it works fine.