lost preview picture of video files in icon

When I open a folder with vob or avi, I use to see a picture of the video. This is now gone. I changed the size to different sizes tried switching from details back to video size and still no picture.
I hope I have explained the problem so that you understand what I mean.
Why have I lost this preview picture of my videos and how can I get it back?
Can anyone help? I always receive helpful advise from this site and I am hoping I will again.
Thank you