If you look at the attached 3d scene you'll see that there are two ways to do the same effect.

The downstairs, if modified Blur node by clicking on "relative", changing the values ​​of X and Y, and clicking again on "relative", you will see that the node stops working. Even if you change the type of filter and back to the previous one, does not work.

The above doing the same works.

With Blender 2.64a from blender.org not work.

Blender version: 2.64.9 r52320 (builder.blender.org)
OS: Linux 64 bits (Kubuntu 12.10)
Graphic card: NVIDIA GTX 560Ti