Primefaces Autocomplete itemSelect event is not fired - jsf

Question

In my JSF application, Primefaces autocomplete itemSelect event is not fired. I double checked and it is inside a form element. I can not figure out why it is not fired.
I have listed code below.
<p:panelGrid columns="1" >
<p:outputLabel value="Medicine/Device" ></p:outputLabel>
<p:autoComplete forceSelection="true" id="acStock" value="#{pharmacySaleController.billItem.pharmaceuticalBillItem.stock}" converter="stockCon" completeMethod="#{pharmacySaleController.completeAvailableStocks}" var="i" itemLabel="#{i.itemBatch.item.name}" itemValue="#{i}" >
<p:column headerText="Item">
<h:outputLabel value="#{i.itemBatch.item.name}" ></h:outputLabel>
</p:column>
<p:column headerText="Rate">
<h:outputLabel value="#{i.itemBatch.retailsaleRate}" >
<f:convertNumber pattern="#,##0.00" ></f:convertNumber>
</h:outputLabel>
</p:column>
<p:column headerText="Stocks">
<h:outputLabel value="#{i.stock}" >
<f:convertNumber pattern="#,###" ></f:convertNumber>
</h:outputLabel>
</p:column>
<p:column headerText="Expiary">
<h:outputLabel value="#{i.itemBatch.dateOfExpire}" >
<f:convertDateTime pattern="MMM yyyy" ></f:convertDateTime>
</h:outputLabel>
</p:column>
<p:ajax event="query" update="lstSelectItems" ></p:ajax>
<p:ajax event="itemSelect" listener="#{pharmacySaleController.handleSelect}" update="txtRate txtQty txtVal" ></p:ajax>
</p:autoComplete>
</p:panelGrid>
<p:panelGrid columns="1" >
<p:outputLabel value="Quentity" ></p:outputLabel>
<p:inputText id="txtQty" value="#{pharmacySaleController.billItem.pharmaceuticalBillItem.qty}" >
</p:inputText>
</p:panelGrid>
<p:panelGrid columns="1" >
<p:outputLabel value="Rate" ></p:outputLabel>
<p:inputText id="txtRate" value="#{pharmacySaleController.billItem.rate}" >
</p:inputText>
</p:panelGrid>
<p:panelGrid columns="1" >
<p:outputLabel value="Value" ></p:outputLabel>
<p:inputText id="txtVal" value="#{pharmacySaleController.billItem.netValue}" >
</p:inputText>
</p:panelGrid>
<p:commandButton value="Add" action="#{pharmacySaleController.addBillItem()}" ></p:commandButton>
</p:panelGrid>
Controller
public void handleSelect(SelectEvent event){
calculateRates();
}
public void calculateRates() {
System.out.println("calculating rates");
if (bill == null) {
System.out.println("bill is null");
return;
}
if (billItem == null) {
System.out.println("billItem is null");
return;
}
if (billItem.getPharmaceuticalBillItem() == null) {
System.out.println("pharmaceutical bill item is null");
return;
}
if (billItem.getPharmaceuticalBillItem().getStock() == null) {
System.out.println("billItem.getPharmaceuticalBillItem().getStock() = " + billItem.getPharmaceuticalBillItem().getStock());
return;
}
System.out.println("cal error check finished");
//
System.out.println("billItem.getPharmaceuticalBillItem().getStock().getItemBatch().getRetailsaleRate() = " + billItem.getPharmaceuticalBillItem().getStock().getItemBatch().getRetailsaleRate());
billItem.setRate(billItem.getPharmaceuticalBillItem().getStock().getItemBatch().getRetailsaleRate());
billItem.setDiscountRate(calculateBillItemDiscountRate(billItem));
billItem.setNetRate(billItem.getRate() - billItem.getDiscountRate());
billItem.setGrossValue(billItem.getRate() * billItem.getQty());
billItem.setDiscount(billItem.getDiscountRate() * billItem.getQty());
billItem.setNetValue(billItem.getNetRate() * billItem.getQty());
}