Rooting Docomo S3 (SC-O6D)
There is a binary in /system/bin which continuously  checks for root and immediately remves it again, as soon as root is detected. Chainfire root tools used to have an automated removal tool affecting this security binary. You can also do it by hand.

I do not know it by hard, the name of the binary. But you could extract the auto-root in the link below and you will know it.
Or you can simply use it.

http://download.chainfire.eu/248/CF-Root/CF-Auto-Root/CF-Auto-Root-d2dcm-sc06d.zip

Have fun!