自定义下拉和组合框问题？

I have a form.In that iam having my customized data dropdown.In one case combo box is below the custom data dropdown.when i click the dropdown th div iam caling has to come top of the combo box.It is not coming on the top the stack.the combo box and div are overlapping. Any Suggestions to solve these problem?I have a form.In that iam having my customized