Question: Before I upgraded to Ubuntu 12.04, thumbnails on video files worked nicely, after the upgrade, thumbnails aren't generating on video files and only on video files, most of them are .mkv files. I have some .pdf files and they all have thumbnails. I have tried to clear the thumb cache, but no luck.

Answer: 
Please ensure that the necessary packages are installed by running the following command in the terminal:

```
sudo apt-get install gstreamer0.10-plugins-bad gstreamer0.10-plugins-base gstreamer0.10-plugins-good gstreamer0.10-plugins-ugly totem libxine1-ffmpeg

```

Also, please delete the thumbnails that failed to load by running the following command:

```
rm ~/.thumbnails/fail/gnome-thumbnail-factory/*

```

In Ubuntu 13.10, the folder changes to: .cache/thumbnails, so you should delete those failed files instead:

```
rm ~/.cache/thumbnails/fail/gnome-thumbnail-factory/*

```
I had to re install my ubuntu to get the video thumbnails working again, it was annoying. I couldn't fin any other way.