Vector Blur Node not blurring

I have a bunch of boxes flying apart in Cycles, and I'm using the Vector Blur node for motion blur. Nothing's blurring.

It's the simplest compositor setup: RenderLayer Image/Z/Speed goes to Vector Blur node Image/Z/Speed, Vector Blur node goes to Comp.Out and Viewer.Out. I've tried all sorts of settings on the VB node, but nothing changes. What have I not set up?

• Is it possible you could upload your .blend? – gandalf3 Oct 20 '15 at 21:43
• Trying to remember how (blush)... Done. – OroNZ Oct 20 '15 at 21:46
• @Carlo Be careful when re-uploading .blends. In general it's probably not as important to upload question .blends compared to answers. – gandalf3 Oct 20 '15 at 22:13
• Uh, sorry @gandalf3! I'll carefully read that post. Thanks for the advice. – Carlo Oct 20 '15 at 22:18

This is because the vector pass can't "see" through transparent (glass) objects; in this case all it sees is the lens object, which is remaining motionless relative to the camera.