My father in law suffers from colitis, which developed after the Parkinson's onset. I think that he also has trouble digesting the drugs he is given limiting the effectiveness.
In the last month his doctor gave him parcopa, which dissolves on the tongue(overcomes digestive issues), and his symptoms showed tremendous improvement immediately. He is now walking, talking and thinking better than he has for a few years. Something to consider if the medicine seems to not be working.