LD_PRELOAD not preloading

The sequence of commands below shows that the intended library was not preloaded. This occurrended suddenly, while it was successfully loaded previously. Any guess? Thanks

francesco@vaio:~$ ldd /bin/ls
linux-vdso.so.1 (0x00007fff34150000)
libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007f311f83d000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f311f67c000)
libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f311f608000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f311f603000)
/lib64/ld-linux-x86-64.so.2 (0x00007f311faa3000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f311f5e2000)

francesco@vaio:~ls
libX11.so.6 new_old_RCM old_RCM old_RCM_linux_64bit RCM

francesco@vaio:~$ LD_PRELOAD=libX11.so.6 /bin/ls
libX11.so.6 new_old_RCM old_RCM old_RCM_linux_64bitRCM

francesco@vaio:~$ ldd /bin/ls
linux-vdso.so.1 (0x00007fff5ffd7000)
libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007f9141281000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f91410c0000)
libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f914104c000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f9141047000)
/lib64/ld-linux-x86-64.so.2 (0x00007f91414e7000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f9141026000)