OCM -Sales order Process problem

Hi:

I am testing OCM process for sales order..But no change at all when I execute COCM,I didn't see any change object there..It's empty..

1.Create Sales order

2.MD50 run MRP for this sales order and convert it to production order,then release it

3.Change the sales order(change description and order priority)

4.COCM but no change object displayed...

what's wrong with it?am I wrong when change the sales order?thanks very much..