The needleplate converter on the Horizon had some problems at the beginning. Something with the spring. I would ask the deal if your machine was one of the ones that was issued a replacement needleplate converter.
My LQS's demo machine had that issue and they were given by Janome a free redesigned needleplate.
If your question is just about the converter itself and how and when to hit the button depending on where the bobbin thread is, I've had the same issue, it's operator error. LOL. I just keep hitting the button until it clicks into place.