Hi David,
Aftermodify is triggered if any of the element within the Node is modified. If you modify some element within the child node of Root node then it won't trigger the after modify of Root Node.
Also if you change the value and put back again the original value, Save would be enabled but system would not consider this as a change unless the value is really changed.
I cannot think of any other reason on why it won't be triggered.
Regards
Vinod