Why doesn't zcash-miner show all my GPUs?

I've installed zogminer and have had some success with it. My rig has six RX480 8GB cards. For the first test run I only used 4. I'm trying to get all 6 running now, however, the output of zcash-miner -listdevices only shows 4 of the 6. Yet each GPU is lit up with the fan spinning. All power connections should be correct.

miningrig:~/zogminer$ ./src/zcash-miner -listdevices
Number of Platforms: 1

OpenCL platforms.
FORMAT: [platformID] platformName
[0] AMD Accelerated Parallel Processing

OpenCL devices.
FORMAT: [platformID][deviceID] deviceName
[0][0] Ellesmere
CL_DEVICE_TYPE: GPU
CL_DEVICE_GLOBAL_MEM_SIZE: 8258945024
CL_DEVICE_MAX_MEM_ALLOC_SIZE: 4244635648
CL_DEVICE_MAX_WORK_GROUP_SIZE: 256
[0][1] Ellesmere
CL_DEVICE_TYPE: GPU
CL_DEVICE_GLOBAL_MEM_SIZE: 8516612096
CL_DEVICE_MAX_MEM_ALLOC_SIZE: 4244635648
CL_DEVICE_MAX_WORK_GROUP_SIZE: 256
[0][2] Ellesmere
CL_DEVICE_TYPE: GPU
CL_DEVICE_GLOBAL_MEM_SIZE: 8516612096
CL_DEVICE_MAX_MEM_ALLOC_SIZE: 4244635648
CL_DEVICE_MAX_WORK_GROUP_SIZE: 256
[0][3] Ellesmere
CL_DEVICE_TYPE: GPU
CL_DEVICE_GLOBAL_MEM_SIZE: 8516612096
CL_DEVICE_MAX_MEM_ALLOC_SIZE: 4244635648
CL_DEVICE_MAX_WORK_GROUP_SIZE: 256

Some things I'm wondering:

  1. Where are my other two GPUs?

  2. Should I be concerned that one card shows a lower CL_DEVICE_GLOBAL_MEM_SIZE value? These 6 cards are identical so all values should match right?

  3. Any PCI settings I need to configure in my BIOS? I'm using risers for all the ports. My GUI-based BIOS only shows 2 of the 6 cards lit up out of the 6 slots, so that also feels like a red flag as-is.

  4. In general, are there any handy tools I should install on ubuntu linux to get GPU information? Right now I'm just familiar with clinfo and zcash-miner -listdevices to enumerate devices. I'd like to see something that shows detailed info for each GPU: manufacturer, revision, firmware version, bios version, memory, etc. All the good stuff.