When the nominal record module is open, you can select a nominal code then you click into 'Activity' to interrogate all movements within that code. If you want to look at a new code, you have to click away into the 'details' field > select the new nominal > return to the 'Activity' field in order to see the activity in that new code.
It seems logical to just allow the nominal to be selected from the dropdown box that is greyed out in the 'activity' field.