This is something I've noticed on my previous iPhones as well as my current 6 Plus. It has happened with Apple wall blocks and cables, as well as MFi certified options. Sometimes, i plug my phone in the 2.1 amp charger, and noticed it isn't charging at normal speed (about 1 percent a minute). Sometimes, I have to unplug it and plug it bag in, or unplug the USB cable from the block and try again. I recently purchased a USB power meter and noticed that when this happens, the amps will either be stuck at about .46-.47 amps, or .96-.97 amps. At normal speeds, the amps are about 1.86a. Anyone know why this happens?