Which method are you trying to put the BPM against? ttOrderDtl should have values in it.
If it can’t be done with a Method Directive, then try a data directive and update the OrderDtl table when you make the change in the OrderRel. I believe the next method called when you perform a save on the sales order is the GetByID so the data should be retrieved from the database and update the form. A trace should show you the execution order, if your not using it try out the trace differ tool.