Is this a service you have written? What does it return if it is called without its parameters?
If it doesn't return anything, SharePoint may be assuming that the service is down, so it doesn't send anything through the connection.
You might have your service return "something" if is called without parameters (or with null parameters), such as some kind of prompt text. (Ideally the null result set will have the same schema as the filled results.)
- Woody -