Populating values in drop down based on other drop down

Hi,

I had created a custom component and had created two drop down fields for country and region.My requirement is if I select the country field then the region field should populate only the regions of that country field.It is working for first time.But if I select another country for second time then the values in the region field are not getting changed.Can anyone please  tell .

Regards

Vamsee