Grass Trees Need Help

HI All,
I will try and make a very long story as short as I can. I am a garden novice & I had my back yard landscaped by a so called "professional" approx 5 months ago. As part of that were 4 Johnsoni Grass trees approx 1.2m in height, (that it turns out I paid way too much for $1000 each). The landscaper and his very questionable supplier will not supply tags or evidence of registration, and have not been helpful at all with how to care for my new trees. I have been watering them at a rate of approx 5lt per tree per day (semi free draining soil), I have tried Seasol and also the brown sugar trick. 1 of the trees in particular had a black center at planting with no new growth and was told "that it had recently flowered, don't worry about it" this tree quite quickly appeared to die off and was instructed to trim off all dead leaves, this tree has gotten worse by the week (still with no new growth) and I have discovered today that central leaves are pulling out easily. Do I have any form of recourse with the Supplier/Landscaper? They have their money, but I am left with a supposed dead tree. I just really don't know where else to turn. I did my homework and chose a reputable landscaper to give me my dream backyard. Please help, I really just don't know what to do now.