Getting back to my previous post, I suspect you're trying to preview files you haven't downloaded enough of to be able to preview. That's why I suggested you experiment with the Launch button from the library window on a file you have already finished downlding. If that works then it's probably the 1st reason.

You keep saying within LW, remember it's not LW that actually plays the movie. It is only launched thru LW & it's the video program that actually plays it. Perhaps the video program is hidden behind LW b/c you keep clicking on LW. It can take several seconds for the video program to open & then load the movie.