Irctc Doubt

Namaskaram, Can anyone let me know whats the irctc preference pattern? Even when i try to book for my parents who are >60 yo, sometimes, we couldnt get lower berth, though there are around 200+ tickets at the time of booking. Im a 30 yo male, yet i never get side lower. And many times we got the seats just beside the kalava. Would appreciate some info on this and some cheats. Thanks.