我无法使用su命令以root用户身份登录，但可以使用SSH登录

我怎么可能无法通过rootsu root或身份登录su（出现错误的密码错误），但是却可以通过ssh root@localhost或ssh root@my_local_IP使用相同的密码登录？

我正在使用CentOS 6.4。

更新1：

cat /etc/pam.d/su

给出：

#%PAM-1.0
auth        sufficient  pam_rootok.so
# Uncomment the following line to implicitly trust users in the "wheel" group.
#auth       sufficient  pam_wheel.so trust use_uid
# Uncomment the following line to require a user to be in the "wheel" group.
#auth       required    pam_wheel.so use_uid
auth        include     system-auth
account     sufficient  pam_succeed_if.so uid = 0 use_uid quiet
account     include     system-auth
password    include     system-auth
session     include     system-auth
session     optional    pam_xauth.so

Update2：

$ sudo grep su /var/log/secure | grep -v sudo

给出：

Feb 23 13:12:17 fallah su: pam_unix(su:auth): authentication failure;
logname=fallah uid=501 euid=501 tty=pts/0 ruser=fallah rhost=  user=root

重复约20次。