Strange behavior on a form

Hi to all, I need a little help to understand a strange (for me that I'm a total beginner :-) ) behavior in Navision 2009.

I have a form that has several lookup fields that opens a list of child record to select one.
The strange behavior for me is that when I click on the upper arrow the lookup correctly opens but when I select the object in the opened window the field on the starting form is not updated.....I've double checked all settings (as far as I know) on the form and on the specific field but I can't find anything......The starting form never updates....this is only for the field that opens a lookup form to select another objects.
All other fields (option list with enumerables, text fields......) works well.
I'm fully aware this is a weird question but can someone help me to understand the behavior.

Thank you for your feedback
Fabrizio