A couple of people have seen this before. It usually means either the IC or potentiometer is faulty, or the board is a poorly designed clone. I've seen cheap clones that don't actually wire up the potentiometer correctly which results in almost 12V on the Vref - at which point the IC will be completely fried.
Best case scenario, return them as faulty to the people you bought them from and either get a replacement or find another place to buy them from. If you bought clones from somewhere like eBay, I'd highly recommend you buy from a reputable store like ProtoPIC (UK), CoolComponents (UK), direct from Pololu (US+Int), etc. You are much more likely to get something that works even if it costs a fraction more - plus if they don't work it is far easier to contact the store and get them replaced.
I've only seen 2 faulty boards out of the 250 or so that I have bought from Pololu and ProtoPIC. Both of those were replaced without any hassle.
Also, could you take a close-up of the driver boards for me.