Plum tree damage

Alan starts with ...
We have a large plum tree which gives blossom in the spring and shade for the granny flat in summer.
In the past 2 or 3 years something has been damaging the tree and its looking like it is ready to die. I have taken steps tp keep possums off but no luck. I'm still not sure if this is possum damage or some insects taking advantage of a weakoned tree. Any ideas?