Jo, can you remind us of her age, history, and how long she has been with you???
I feel that maybe little Truly was a little confused when she came to you, and very unsure of things - which often leads a dog to 'shutdown' wherby they are still anxious/afraid of new things but there bodies are under so much stress they are unable to react. Often, once they feel a little more at ease, there bodies are then able to react, which may be what you are experiencing with Truly.
For now, I'd work on having her go to a safe place when you have visitors, often, behind a babygate can be a good idea, where she can see, hear and smell you all. Alternatively you could have her on lead and ask her to 'settle' when they come but experience has taught me that the safe place often works better. As others have said, ask new people to ignore her completely, so she doesn't feel threatened.
Is it the eye contact, voice or body language that sets her off?? Or a combination of all three???
Also, whats the rest of her body doing when she does go off on one??? And the barking does it include growling?? Is the bark high pitched???
Have you tried clicker trainign with her??? It might be worth a go to try and build up her confidence and reward her for focusing on you and being calm