ETHERNET AND USB SHIELD compatiblity

Hi,
I am using an USB shield and Ethernet shield
I have re-wired the SS pin on the USB shield and the USB shield works fine without the need to connect to pin 10.
To ensure that my ICSP pins are connected, I replaced my Ethernet shield with double sided male connectors and then used jumpers wires on the mega2560 and the USB shield to ensure physical connectivity between the ICSP connectors.
I still seem to have trouble with the SPI communication on my USB shield
Any reason why?