How can I preview video files in quicktime

How can I preview video files in quicktime by click on the space bar like i use to with leopard?

I like to pull up all of my video files and arrowing down and hitting the space bar and previewing them.

I cannot do that with all of my video files just some of the video files are able to be previewed.

Some of the files show a thumb of the video and some just show the quicktime logo are other logos

I am running the latest Lion

Thanks.