Valuation catagory greyed out when creating material for new plant

have just created a new Plant by copying another Plant.

When I try and create a new Material for the new Plant/Material combination, the valuation catagory is greyed out, this stops me from being able to input data so the material can have a split valuation.

When I create the same material for the Plant that was copied the Valuation catagory is available to input so split valuation is possible.

Can anyone help and point me to where the valuation cataory is determined so i can input data so the material can be given a split valuation. This must be possible since the Plant that was copied let split valuation.

Regards