Because you might gave 3-4 characters only so its looks simple . always try to set with atleast 6-9 letters including uppercase and numeric number .
It doesnot mean that password chnage didnt happened. It will change password successfully but not a secure .
[root@tstvm ~]# passwd sai
Changing password for user sai.
BAD PASSWORD: it is too short
BAD PASSWORD: is too simple
Retype new password:
passwd: all authentication tokens updated successfully.