Hi Suzanne,
Welcome to our community! I am afraid I can't really offer any more input, it sounds like you are handling the trees properly, however the fact that their conditioning is worsening isn't a good sign. Are all 15 of them looking the same or are some of them greener than others? Have you had much precipitation recently? In your back yard, where do you have them planted - do they get plenty of sun? how is the soil in the area they are planted in?
Again, it is hard to say what's going on with them, if all 15 of them are browning and getting worse after being green, it could be a soil, climate or some type of disease issue.