This is the problem I ran into with the monoprice hub and why I had to come back and issue a correction to 6 units instead of 7 being supported on them, at least in my case they were just barely short on the power, or the block erupters used just barely too much and then I'd get them randomly not found when starting my miner or some started showing 100% HW errors, as soon as I dropped 1 erupter off each of the hubs then they all started working flawlessly.  So make sure you have enough power.