Combo box choice disappear?

hello allI just upgraded and I need help

I have a form with a combobox when users choose an item from that combo box the rest of the fields are calculated upon this chice ,this was happening very well in r5

now in 6 when I choose any itenm from the combobox it doesnot keep the choice ,the combobox refreshes immediatly and turn back empty so the rest of the fields are not calculated

any idea please

thanx