When I put the car in reverse it makes a noise as if the sensors are active, but then the sensors don't pick up anything.

Where do I start? Will vag com give me any ideas?

A VCDS/VagCom scan would highlight any failed sensors and with the measuring blocks you should be able to see the measurements from the individual sensors (assuming they are working) 

The only thing I would add to that comment, is to make sure they are factory fitted sensors, and not aftermarket units fitted.

 

A few years ago, it was a really popular and cheap aftermarket mod to fit, the only downside was that VCDS does not acknowledge their presence on the car, I have them on my vrs, and they are just a standalone unit that picks up a live and earth from the rear lighting loom.

 

Are they mounted in the black rub strip on the bumper, or are they colour coded to the main bumper panel?.

 

If they are factory fit, then Nige8021 is 100% spot on with his post.