There is a known issue in the GCPO library that can limit the number of notes that will play back at one time, regardless of the polyphony setting in the ARIA Player. This will cause some notes to drop out as more sustained notes are played at once. This problem affects both Mac and PC installations of GCPO.
To minimize this problem, please use this article to update to the most current version of the ARIA player. With ARIA 1.620 and higher, you can set the polyphony levels individually for each slot.