root password is not working for su in terminal

I had the same problem and none of the above replies helped. In my case, I could login as root, sudo worked, but not su.

It turned out that /bin/su had somehow lost the suid permission.

I corrected that with chmod u+s /bin/su, and then su worked again.

Probably you have locked the root user by too many wrong authentications. Try

usermod -U root

to unlock the account.