>>I thought you said one that wasn't working was bound to an object? :
"The only difference I can see is the data in the first case is coming from a datatble, while in the second case it's coming from properties in an object "

Sorry for the confusion, Viv. In both cases, the textbox is bound to a datatable.
In the case that works, the datatable is being updated from another data table.
In the case that doesn't work, the datatable is being updated from an object.