Each rep has an impact on the components of handling time and, therefore, on the raw data used for forecasting. Your software needs help. Forecasting software takes much of the laborious work out of capturing and crunching historical data. But it doesn't think for you. It doesn't know that average handling time will be affected by changes being made to systems or that a new service is being released next week.
Accurate forecasting goes beyond the call center.
One trip to a call center the morning after an unannounced marketing campaign will bring the painful realization that the marketing folks need to be a part of this process. So do the people in legal who change terms and conditions and the people in IS who are making system changes and anybody else that impacts a call center's work load. Your agent groups must be simple enough to manage. Some call centers have skills-based routing environments that are so complex they have only a vague idea of where calls will end up. If a center needs the right number of Norwegian-speaking agents trained on a special support line and in place at the right times, management needs to forecast those calls.