Declare and <DllImport> in VB.NET have different results

Question

I've been trying to call a login method of an unmanaged DLL.

If I use Declare the login fails.

Private Declare Function Login Lib "dllCore" (ByVal lpName As String, ByVal lpPassword As String) As Int32

Login ("Steve", "123456") ' THIS FAILS TO LOGIN ALTHOUGH THE PARAMS ARE CORRECT

If I use DllImport, it works !!

    <DllImport("dllCore.dll", 
                EntryPoint:="Login", 
                SetLastError:=True, 
                CharSet:=CharSet.Unicode, 
                ExactSpelling:=True, 
                CallingConvention:=CallingConvention.StdCall)> 
        Private Function Login(ByVal username As String, ByVal password As String) As Integer
        End Function

Login ("Steve", "123456") ' NOW WORKS 

Has anyone any ideas why I get this behaviour ??