How to charge an android device with a buck converter
Measure the current draw. My guess is you're only getting ~100mA. You could try shorting the USB data pins, as that is the USB 2.0 standard for a dedicated charge port. However, you know what they say about standards. Not all phone manufacturers will recognize a DCP, and worse, others expect some other weirdness (looking at your Apple and Samsung) on the data pins to fast charge.