Back bean method not called while using single form for two h:selectonemenus and command button

my back bean method is not called when i execute the following code, hope i will get the solution.

 <rich:panel style="width: 750px" header="Manage Content">
 <h:form>

 <h:outputText value="Select Division's Id : "/>
 <h:selectOneMenu id="divId" value="#{pageDataBean.selectedDivId}" style="width: 150px" >
        <f:selectItems  value="#{pageServiceBean.retriveDivId}" />
 </h:selectOneMenu>

 <h:outputText value="Select Content Type : "/>
 <h:selectOneMenu id="firstDropDown" value="#{pageDataBean.selectedBlockType}" style="width: 150px">
        <f:selectItems  value="#{pageServiceBean.blockType}" />
        <a4j:support event="onchange" reRender="secondDropDown" ajaxSingle="true" />
 </h:selectOneMenu>

<h:outputText value="Select Content : "/>
<h:selectOneMenu id="secondDropDown" value="#{pageDataBean.selectedBlock}" style="width: 150px">
        <f:selectItems value="#{pageServiceBean.blockName}" />
<开发者_如何学JAVA;/h:selectOneMenu>

<h:commandButton value="Add" action="#{pageServiceBean.addContentToDiv}" style="width: 75px">
</h:commandButton>
</h:form>
</rich:panel>