Are you using a fast charge/adaptive fast charge wall adapter? If not, fast charge probably won't work? Also you wanna make sure you're using the proper usb-c cable as this can also affect fast charge. I use a Samsung adaptive fast charge adapter in combination with a usb-c 3.1 cable.
You can find them at Wal-mart, Best buy,ect. Your best bet would be Amazon, there you can find them in lengths you won't find in the locations I mentioned