Our company has faced this USB compatibility issue as well. We make a line of solar USB chargers (with rechargeable AAs) and have had to tweak our circuit designs several times to allow the charger to charge most devices that can be charged via USB. Notably, Apple (requires some resistor magic between a few USB pins) and several smartphones ignore the USB standard. Sigh.
Anyway, it's a challenge for those of us trying to make a USB charger that will charge (most) anything when phone makers don't follow the standards published.
Let me know if I can help with the USB questions.