Alt + SysRq +
R - takes keyboard and mouse control from the X server. This can be useful if the X-Server crashed, you can change to a console and kill the X-Server or check the error log.
NOTE:- The documentation refers to this key's task as "Turns off keyboard raw mode and sets it to XLATE", but I suppose it's safe enough to assume that it takes back control from X.
E - sends SIGTERM to all processes except init. This means that an attempt is done to end the current processes except init, safely, e.g. saving a document.
I - sends SIGKILL to all processes except init. This means that all the processes except for init are killed, any data in processes that are killed will be lost.
S - writes all data from the disc cache to the hard-discs, it is a sync and is necessary to reduce the chances of data corruption.
U - remounts all mounted filesystems read-only. After using this key, you can reboot the system with Alt+SysRq+B without harming the system.
B - restarts the system without making steps to ensure that the conditions are good for a safe reboot, using this key alone is like doing a cold reboot.
Try without pressing R at begining of your sequence
or just do it in another seq.. give a litle time between those keystrokes and do this..
Alt + SysRq + R S E I U B
and better you enable Magic SysRq by entering..
echo 1 > /proc/sys/kernel/sysrq