CACTI - Network Monitoring Tool

Hello, I have installed cacti one month back. When i see the daily, last 2 hour, last day and last week reports , i am able to see the correct graph. But when i am trying to fetch the last 2 weeks , last month and above reports , the graphs is incorrect. eq: the graphs show the utilization above 8 MBps for daily and last week reports and when i see the graphs for one month or above , the same utilization shows 6 Mbps. I need some one who can guide me and as well as implement the correct setting for the cacti server.