Cannot move cursor in combobox using mouse at form in panel

Hi all,

I create search form and it's run well, there are some combobox, and when click a combobox using mouse the cursor move normally, but when I open the form in panel, the cursor only move to textbox using click mouse, but cannot mouse to combobox using clicke mouse, using tab key or enter key, it normally moved. I am using visual studio 2010. Please help. Thanks before