UPDATE - Just tested on 2 rigs that was only hashing with 6 vs 7 GPU on 1 and 4 instead of 6 under Optiminer.  When I switch to Claymore Zec, it works for all GPUs.  However Claymore is slower so we definitely need to get Optiminer to be more compatible.   So doesn't appear to be a USB issue as I tried it using USB 2.0