Employee productivity is too complex. It depends on motivation of workers, culture of interdependence and cooperation, quality raw materials, quality of the machines that they use, leadership qualities of the managers, their own job knowledge, remuneration as per market standards and so on.
Next thing for you is to do the time and motion studies. Study the cycle time for each process. Find out whether any process is redundant. Find out whether any resources are idle during working hours.
Next thing is multi-skilling. This is yet another method of improving productivity.
Lastly, encourage suggestions from the employees. A good many suggestions come from the employee themselves to improve the system or processes.
All the best!