rebate on the basis of scale

hi gurus ,

in my co. rebate is calculated on the basis of scale viz

0-10ton = rs55

11-20ton=rs.100

i have maintained the record in rebate agremenent ,condition record.To my surprise system is only taking first scale by default. its not using scaling inspite of scales record.

can anybody advice me to solve this scenerio.

rgds,

Aman