I'm having trouble with overclocking my Gigabyte RX 480 8GB. I can't overclock it without having it crash under load. I've contacted Gigabyte support and tried many different things, but I don't really know what else to do but ask here. I'm running the latest software for it, and sometimes the WattMan will crash upon opening. I'm running an i5 6600K and a Z170 gigabyte board with 32GB of RAM and a 650 W power supply.
The Wattman crashing issue is currently being looked into. Not every GPU is guaranteed to be overclockable, if altering the voltage doesn't help there isn't much else you can do other than asking for a replacement assuming the manufacturer would accept that as a replacement reason.