Substitute items in Formula AX2009

HI

I am trying to get the substitute items working in a formula, but I cannot get the alternate item to be used in the planned production order formula.

Steps taken:

1. In item formula added alternate item with priority 2, and plan group "swap", and nil quantity
2. In formula updated original item with priority 1, and plan group "swap", approved and activated formula
3. Made sure no stock of original item, and checked sufficient stock of alternate item
4. Ran master planning for item
5. check formula on planned production order

Result, original item is still in formula on planned production order.

Anyone know what step I have missed here?

Cheers

Steve