passwords for sharing not working

I have an iMac and a MacBook Pro, both working on OS X 10.7.5. Each of them has a different password for the administrator account that I use with it. Both of these passwords work properly when I am installing software or looking at passwords in the keychain, etc. But neither works when I am attempting to "contact" the other one for sharing. Additionally, neither works when I attempt to change the User password. This means I have had to shuttle across the room with a thumb drive to transfer files. It seems that there should be something I could do to remedy this situation. I'm not sure exactly when this began. It was not always the case.