C# - Cannot Select The ComboBox Item

I have two Comboboxes where the second one is dependent upon the first one.

When the SelectedIndexChanged event of the first Combobox fires then the second Combobox will be enabled.

After the event, the second Combobox is enabled but I cannot select the ComboBox item.

EDIT

I use Dev express Tools

First Combo I load in Page_Load Event

I use Server Side code:

protected void ASPxComboModule_SelectedIndexChanged(object sender, EventArgs e)
{
LoadSecondCombo();
LoadSecondCombo.Focus();
}

There is no problem in loading, but I'm unable to select 2nd combo item.