WindowsAutomation fails when there is no open session

I wrote an automation program which automates another GUI application using the WindowsAutomation feature in .NET 4.0. I do not know the exact technical terms to describe this situation, but ...  it works fine if the programs are actually visible (i.e., I logged in on a real machine, or a remote desktop session is kept open) but it fails if they are not visible (for example, there are two users: A and B, and the programs are running on user A's session, and currently user B is using the computer on the monitor.)

Is this a common problem, or just the behaviour of the particular application I am automating?