With Server Moves and Database Restores from one Database Server to another, it is easy for User Accounts to get "orphaned" where their associated SID (GUID) no longer matches the deployed Server. To fix this, you need to reset or re-sync the Accounts. This is a good solution I found on Stackoverflow which works rather well.

Resync-User-Accounts.sql

-- Source: http://stackoverflow.com/questions/19009488/the-server-principal-is-not-able-to-access-the-database-under-the-current-securi

USE [your database]
GO

-------- Reset SQL user account guids ---------------------
DECLARE @UserName nvarchar(255) 
DECLARE orphanuser_cur cursor for 
      SELECT UserName = su.name 
      FROM sysusers su
      JOIN sys.server_principals sp ON sp.name = su.name
      WHERE issqluser = 1 AND
            (su.sid IS NOT NULL AND su.sid <> 0x0) AND
            suser_sname(su.sid) is null 
      ORDER BY su.name 

OPEN orphanuser_cur 
FETCH NEXT FROM orphanuser_cur INTO @UserName 

WHILE (@@fetch_status = 0)
BEGIN 
--PRINT @UserName + ' user name being resynced' 
exec sp_change_users_login 'Update_one', @UserName, @UserName 
FETCH NEXT FROM orphanuser_cur INTO @UserName 
END 

CLOSE orphanuser_cur 
DEALLOCATE orphanuser_cur