Silver Birch Tree Problems

Enarkay

We live in South Australia and have just moved into a property which has many silver brich trees planted by the previous owners over 25 years ago to aid shading of the house in the summer. Several of these trees do seem to be showing I think signs of stress and I am unclear on what we can do to help them. 3-4 of the trees have leaves that have mostly turned yellow and one tree seems to have dropped most of its leaves altogether, (this started while we were still in summer time) while most of the other silver trees are still fully green. I know we're into Autumn and would expect the leaves to be turning at this time of year but seems strange that they are at different stages. Trees are well mulched and we have the drippers on for 3-4 hours every week with additional watering during some of the very hot weather in the summer. Can anyone provide any advice of guidance on how to look afet the trees and keep them healthy.