Pursuant to Geoffrey Crawley's recommendation on the Paterson Web site, I again exposed a roll of Acros 100 with an exposure index of 200 rather than 100 and processed it in FX-50 (20 ml of bottle A and 20 ml of bottle B with enough distilled water to create 300 ml). A change I made this time was to add two minutes to his time recommendation of 9 minutes, and after processing it for 11 minutes I can now see more tonality and density but not quite enough.
Yesterday I ordered another FX-50 box from B&H. When it arrives, I will again expose the Acros 100 at EI 200 and develop it for 14 minutes instead of 11 and analyze the results. If there is another gradual favorable tonality increase, I suppose I can assume that the existing bottles are not going bad (and they show no discoloration to indicate that) and for some strange reason Geoffrey's recommendation of 9 minutes simply is insufficient.