Dock has bizarrely stopped charging iPhone

I seem to be having problems with my iPhone or dock.
Bizarrely, the dock I bought now does not seem to charge my iPhone, even though it always has done, did this morning, but suddenly its not.
i always get 'this device is not compatible, would you like to turn on airplane mode?' which i do do, but its coming up with 'charging is not compatible with this device'.
What is to blame? The dock is a Iwantit iw900, the big black one.
How can this have happened anyway, its very odd?