could be compatibility issues, U3S6 are compatible with only selected of motherboards, here is their compatibility list on the asus website:
Maximus III Formula
P7P55 WS Supercomputer
P6T Deluxe V2
Rampage II Extreme
Rampage II Gene
you could try to flash your motherboard bios to the newest version, and then try to update the firmware on U3S6 to the newest version as well. download the latest firmware here: http://www.asus.com/product.aspx?P_ID=lGYmelQ8mJvPtYTv
I also have a U3S6 and I had to deal with compatibility issues at first as well, however, after updating everything to their newest version, everything worked fine.