Why doesn't my combo box dropdown appear when I click the down arrow?

Also sometimes voiced as "why can't I add items to a combo?". You've been caught by one of the commonest newbie Windows programmer problems: you have omitted to resize the combo box drop-down. For some unearthly reason best known to Microsoft, the combo box drop-down defaults to a size of one pixel. Just go into the resource editor, open the dialog and click the combo down arrow. This produces a set of sizing handles so you can re-size the drop-down box.