As well as the contrast, consider the shadow detail in the negs. Was that as required? If not then that is an exposure problem. If yes, then a more active developer would be a diluted print-dev, or graphics-film 'universal' developer.
I vaguely recall that this sort of material was expected to have a short and automated development at a higher than normal temperature - perhaps an increase in temperature (higher than 20c) could be part of your testing. Along with the possible developer and exposure changes, and thinking about changing just one thing at a time