Castlewellan Gold (?)dying.
 

Hi
New here today and quite a garden novice!
I have 2 Castlewellan Gold trees which I bought already cut into balls, the trunks are about 2-2.5ft and the ball about 1-1.5ft. I have them in pots outside my front door, NW facing, and sheltered. I water them regularly. One is fine, the other has started turning brown. It started about 4 weeks ago and seems to be getting worse. I dont know why, I'm assuming not position as only a door width apart, and assuming not lack/over watering as cared for in the same way? Any ideas? and anything I can do to save the tree??
Thanks
Linda