I ran into this issue a few years ago when making a portable USB charger for my android phone. Android does not have that issue, but when researching to build this is where I came up on it.
Check the voltage on the data pins (2 in the center of the USB) as explained below.
Excerpt from the article as to why:
We did some experimenting (see the video up top) and determined that in fact the different voltages/resistances did effect the charging rates
! Using the 2.8V&2.0V setup resulted in a 1 Amp charge rate and the 2.0V&2.0V setup resulted in a 500mA charge rate.
This made us very happy, because 500mA is within the capability of the MintyBoost chip. We redesigned the PCB to allow us to have 4 resistors on the datalines and put two 75K and two 49.9K resistors in each kit. So far we have had no problems charging any of the latest Apple devices. Hooray!
I built 2 of these with my son, bought the circuit boards from Adafruit and bought the parts locally or online, depending. Added a solar panel off of a dead yard light to charge in the sun or pull the NiMh AA batteries out and use a wall charger on them.
Now that the price of LiPo (Lithium Polymer) portable chargers is way the hey down I use them and keep this as a backup.
Hope this helps some.
2006 Sunseeker 3100SS/LTD
2008 Smart Passion Cabriolet TOAD
1986 Honda Helix (Original owner)