Hi Patrick,Thank you for using Just Answer my name is ***** ***** I will help with your question. Did the unit come with its own charging plug? I suspect that the iPhone charging plug does not supply enough current to charge the unit from flat.I will do some reasearch on what current is needed to charge the unit.
Denon recommend a 5v 2A but no greater than 5v 2.4A charger plug I think the iphone plug is only rated at 500mA or 1A if it is a larger plug.
I would try a computer port but again the port may only supply 500mA it would be worth a try
I have found this charger it is 2.4Ahttp://www.currys.co.uk/gbuk/computing-accessories/computer-accessories/tablet-accessories/tablet-chargers-adapators-and-cable/iwantit-i24amwh16-universal-usb-charger-10142685-pdt.html
I agree, it states in the manual it can be charged off a computer but also states they recommend a 2A plugtop. Do you have another charging lead that would fit it. It is a micro usb lead that is used on many mobile phones.After 30mins on the computer I would expect some signs of life. I think you may need to return it as get a replacement.
As you have now tried another lead. I think the best course of action is to take it back. It is strange it worked fine for a bit. I would recommend getting a 2A or 2.4A charger as it will charge it quicker. I do feel now that even if you had a bigger charging current the unit will still fail to work.
I am sorry I could not help more hopefully amazon will be quick in replacing it.