Cannot Access PC In LAN - Get Error Message Logon Failure?

I have three PCs in my lan. (A, B & C).I can access shared drives of B & C from A

I can access shared drive of A from C but cannot access B from C

I can access shared drive of C from A & B.

I get error message "Logon failure: the user has not been granted the requested logon type at this computer".

All the systems are Win 7, A & B are desktop and C is a laptop.I added the C's name in A's "Security Settings -> Local Policies -> User Rights Assignment -> Access this Computer from the Network" too. Still can't access.