RFID + Ethernet shield

Hey there,
I bought my arduino a few weeks ago and from time to a new asset arives.
First I got the Ethernet shield - UNO Shield Ethernet Shield W5100 R3 UNO Mega 2560 1280 328 UNR R3 only W5100.
A few days ago, my RFID kit arrived - LOT MFRC-522 RC522 RFID Kits S50 13.56 .
They work perfectly apart, but when I put the together the RFID refuses to work.The light is on, but no cards/chips are detected.
My initial though was that the pins didnt make a good contact with the shield.Well I tested them all, and it all seems fine.
What am I missing folks?

both uses the same SS pin ?

Changed SS pin , everything works!