But do you have line
shm /dev/shm tmpfs defaults 0 0
in /etc/fstab ?
For example, on my test Plesk 12 VPS server I see:
[root@ppu12-0 ~]# cat /etc/fstab
none /dev/pts devpts rw,gid=5,mode=620 0 0
none /dev/shm tmpfs defaults 0 0
[root@ppu12-0 ~]# cat /etc/redhat-release
CentOS release 6.5 (Final)