so i have an issue with 2 of my antminer U1's, both have stopped hashing

in BFGminer i see  "AMU 9:       |  2.10/ 2.10/ 0.00Gh/s | A:   2 R: 0+0(none) HW:0/none" that's after 3 hours (though it can be after 24hrs the results are still the same)   and the other is exactly the same.

These are on a powered d-link DUB H7 hub, fed off a 350w PSU (so plenty of juice), over clocked to 0981, with a 120mm fan on them. Even if i move these ants onto there own hub, i still get nothing  

has anyone had similar, and is there a fix?

edit, both the heat sinks are tightened up correctly and there is no debris behind them etc

Hi guys, sorry if this has been covered somewhere ( I did a search and I couldn't find an answere)

How many antminer u1's can you have on a usb channel? (Assuming that the ants are on a powered hub)

The reason I ask is I am looking at my old raspberry pi, and wondering how many it can handle?

Thanks in advance