In my case, the query in the console returns the value 16 which, according to the documentation, allows only one sync to the hard disk.
cat /proc/sys/kernel/sysrq
The sync is probably not shown at all and so you don't get that something happens.
To get all functions the value 1 must be entered with
echo 1 | sudo tee /proc/sys/kernel/sysrq.
After that SYSRQ works immediately by typing Alt+Print+R+E+I+S+U+B for me.
But the value 1 or other values must be entered before each time or written permanently into a configfile.