I need some help.

I've connected two Bitfurys to my pi.

When I start cgminer I have a avg. of 2.357Gh/s.

I think thats to slow for two miners.
If I connect only the master I get approx 1.4Gh/s.

Where is the problem?
I have a big fan that cools the miners.

I checked also the Voltages on board and all voltages are in range.

So what can I do?