Scale based rebate agreement on accreued business volume

Hi, could you please show me some light !!!!!!!

I need to configure scale based agreement, which should work across all billing documents for agreement period. eg.

till 10000 TON 2%

10000-20000 TON 3%

20000 and Abave 4%

Here 10000, 20000, 30000 are the total weight bought by customer in this agreement term till this current invoice.

I choosed CALCULATION TYPE :Net Weight

SCALE BASIS : Net Weight Scale

It's not working. Any other suggestion???