RadComboBox: Problem with webservice and postback

[SviPla]

Hi all

I have a RadComboBox:

<telerik:RadComboBox ID="radCmbRegioniFilter" runat="server"
 Filter="Contains" AutoPostBack="true" EnableLoadOnDemand="True"
 onchange="Page_BlockSubmit = false;"
 OnClientItemsRequesting="radCmbRegioniFilter_OnClientItemsRequesting">
 <WebServiceSettings Method="GetRegionAll" Path="~/controls/OnDemand.asmx" />
</telerik:RadComboBox>

When I click on the combobox the data are load correctly.

If I execute a postback and I click on the combobox, the webservice is call but the combobox contains only the selected item and blank in others rows. If I click again on combobox I see all data.

Why?

Thanks