The problem is that, even though your phone is rooted, the 'adbd' server on the phone does not use root permissions. You can try to bypass these checks or install a different adbd on your phone or install a custom kernel/distribution that includes a patched adbd.
Or, a much easier solution is to use 'adbd insecure' from chainfire which will patch your adbd on the fly. It's not permanent, so you have to run it before starting up the adb server (or else set it to run every boot). You can get the app from the google play store for a couple bucks:
https://play.google.com/store/apps/details?id=eu.chainfire.adbd&hl=en
Or you can get it for free, the author has posted a free version on xda-developers:
http://forum.xda-developers.com/showthread.php?t=1687590
Install it to your device (copy it to the device and open the apk file with a file manager), run adb insecure
on the device, and finally kill the adb server on your computer:
% adb kill-server
And then restart the server and it should already be root.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…