Windows Service doesnt receive callback events

I have a Win32 Dll and imported it into my .Net C# Application. This Dll calls some callback functions defined in my app. It works well, but I had to rewrite my app to Windows Service. Now I dont receive the events from Dll. I observed that in service, callback functions run in a separated thread and in app, the main process and callbacks run in a same thread.

Does anybody has any idea about this?