su does not change prompt to # I am trying to root a device.
I have pushed su , busybox to device, 
but on doing "su", it seems that I have no root privilege: 

shell@nemo:/ $ su
1|shell@nemo:/ $

1|shell@nemo:/ $ whoami
shell
shell@nemo:/ $


How can I understand what's wrong with su and how to fix it ?

EDIT:

I am actually trying to root LG W200 device, having real trouble with it. 

EDIT2:

I've set 7777 permission (suid), but it still fails.

chmod 7777 su-arm

 shell@nemo:/ $ ls -al /system/xbin/su-arm
-rwsrwsrwt root     shell      467952 2016-11-20 12:01 su-arm
shell@nemo:/ $ su-arm
255|shell@nemo:/ $ id
uid=2000(shell) gid=2000(shell) groups=2000(shell),1004(input),1007(log),1011(adb),1015(sdcard_rw),1028(sdcard_r),3001(net_bt_admin),3002(net_bt),3003(inet),3006(net_bw_stats) context=u:r:shell:s0
shell@nemo:/ $whoami
shell


EDIT3: I've updated su binary and busybox binary, but each now fails differently:

shell@nemo:/ $ busybox su
su: unknown user root
1|shell@nemo:/ $ su
error: only position independent executables (PIE) are supported.
1|shell@nemo:/ $


Thanks,
Ranchu
You have to unlock the bootloader first. Here is a page to help you do it: 

https://developer.android.com/wear/preview/downloads.html

Here are step by step instructions:

http://forum.xda-developers.com/watch-urbane/development/everything-urbane-root-how-tos-files-t3243975

That page also contains the files you need to flash.

IF YOU ARE NEW TO THIS PLEASE READ THE POST ENTIRELY BEFORE TRYING TO FLASH OR INSTALL ANYTHING. I TAKE NO RESPONSIBILITY FOR ANYTHING YOU DO TO YOUR DEVICE.