Are you using a web browser form? Is your dropdown control populated only when a field's value changes? I'm thinking you might need to set a control's postback settings to 'always' so that it would attempt to send data to server and that the form would render correctly in the browser.
You can quickly test if that's the issue by switching to another view in your form and then switching back to the view where the dropdown is - does it populate when you switch back?