I've faced this problem and can not find why. Here's my code on document ready.
$('#cb-year').combobox({
valueField: 'id',
textField: 'text'
});
$('#cb-year').combobox('loadData', years); //years is json variable
$('#cb-year').combobox('setValue', '2015')
;
When i click combobox, i see '2015' is higligted, but when i run $('#cb-year').combobox('getValue') in browser console, it returns empty string.
if i ('getData'), the data returned seems correct [{id:'2015', text: '2015'}, and so on].
Any clue why ?