ethminer not using GPUs

I'm trying to mine using ethminer on Win10 x64 and 6xRX 480. Ethminer sees all six cards, but I can't get it to use them for mining or benchmarks, even when I specify them through --opencl-devices. It only uses the internal GPU, which is useless. I've tried disabling the internal GPU, and then it says "no GPU device with sufficient memory was found". All GPUs are 8GB, and I am using the "-G" option. I can mine on Minergate fine, but it only uses four of the cards. I'm using the 16.9.2 drivers.

Any idea how I can get ethminer working? Thanks!