New iPhone 4S won't charge via solar lamp

I recently bought an iPhone 4S. I was hoping to charge it via the USB lead from a sun king solar lamp (excellent things by the way), but the phone won't charge.

I originally thought it was a problem with the lamp, the maker did too, but it turns out it charges my old iPhone 5 just fine (other peripherals too). The maker says it should work. Any ideas? The phone is still covered by the warranty, but I'm not sure Apple would agree to exchange it on the grounds that I cant solar charge it! :-)

Oh, I've tried with another lead and it's the same story.

All best,

Tom