Since the users' query seems to be fetching more and more data every day. As the data gets to increase, the amount of information that could be stored in the buffer_cache decreases, since it had to service different users queries. as a result the miss rate increases and draws down the hit ratio. To better serve the problem, refine the user queries and use bind variables. Also as Reddy pointed out, increase the apporpriate parameters to serve the need.

Sam