can't unlock the perference system and my administrator name and password is correct

i am using the mac pro with OS X Yosemite Version 10.10.5

i can't unlock the perference system to make any change and my administrator name and password is correct, i am pretty sure with that otherwise i won't login to my account in the login windows so i don't know what s happening about that, there are 3 different account on my mac, my own account and user and guest which is i don't know the password of them.

please help me!!