Looking at the GDS, it seems there is plenty of room for the nets to route between the macros. There are less than 150 nets to route between the macros, in total.
Where should I start looking to determine the cause for incomplete or inability to fully route?
Problem resolved: In this specific case, the dependent macro was stale. It was operator error to overlook the need to rebuild.
However, it seems the tools did not complain about missing pins and attempted to route regardless.