Updating Price Differentials on Purchase Order

I am using services procurement for all of our Sub contract labor where each vendor has its own PO and each individual sub contractor has their own PO line. On these lines we have set our price differentials which I would like to disable. I am able to uncheck the enable box in the price differentials screen for everyone of the sub contractors except for one; for some reason those fields have been protected against update. All of the other contractors who have charged against their price differentials are able to be updated; I am wondering why I cannot update