Do you mean Antminer U1 or U2?
Perhaps your USB extension cable has too much DC resistance for the power needs of the USB miner. If it runs ok when you connect it directly to the port, but stops working with the extension cable, that is most probably the reason. Try to find another extension cable (a shorter one is likely to have lower losses) or get a powered USB 2.0 hub and use that with your U1/2 Antminer.
Cheers
Mine is U2. My cable's length is 1.5 m.