Can't modify IUM on Sales Order Entry form

In Vantage 803.409C, we want to be able to specify several different units of measure for an order line. I believe that the UM for both "Order Quantity" and "Our Quantity" fields initially populates from the Part table.

I was able to replace the Order Quantity UM field with a combo and define new entries (besides EA). That works fine. However, I can't change the Our Quantity UM (IUM field form OrderDtl). I first tried adding a form event to set the Our Quantity field to the same value as was selected in the Order Quantity UM field. That worked, almost. I could see that the field contents were changed but immediately changed back to the original contents. I tried changing the field's readonly property to false, but it wouldn't allow me to do it.

I then tried to do the same thing as I did with the Order Quantity UM and add a combo linked to the OrderDtl.IUM field. The combo is there but I can't drop it down.

Can anybody offer any ideas as to why I can change the Order Quantity UM value but not the Our Quantity UM value?