Spring model attribute is not populating values

I have a model class with some methods, most of them are populated using ajax(jquery), I have a model attribute on JSP as orderVo, initially when page loads some values remains blank and when i call different methods they keep on filling using ajax. Now issue is, when 1 method is called after form setup(1st GET) my model attribute is not populating with values.

@Controller
@RequestMapping("/order")
@SessionAttributes("orderViewVO")
public class OrderController {

private static final Logger LOGGER = Logger
            .getLogger(OrderController.class);

@Autowired
    OrderService orderService;

//THIS IS THE 1ST METHOD CALLED ON PAGE
    @RequestMapping(value = "{clientId}", method = RequestMethod.GET)
    public String showOrderPage(@PathVariable Long clientId, Model model) {

model.addAttribute("orderViewVO", new OrderViewVO());
        return "/order/orderView";
    }

//I want model to be updated here
    @RequestMapping(value = "/pro", method = RequestMethod.POST)
    public @ResponseBody
    OrderViewVO fetchProduct(
            @RequestParam(value = "brId", required = true) String brId,
            @RequestParam(value = "categoryId", required = true) String categoryId,
            Model model) {
        LOGGER.info("Entry fetchProduct()");
        LOGGER.debug("Branch Id: " + brId + ", Category Id: " + categoryId);

OrderViewVO orderViewVO = null;
        orderViewVO = orderService.fecthProduct(categoryId, brId);
        //this has no effects on model values
        model.addAttribute("orderViewVO", orderViewVO);
        LOGGER.info("Exit fetchProduct()");
        return orderViewVO;
    }

}

JSP -

</script>
<body>

<form:form id="orderView" name="orderView" method="post" modelAttribute="orderViewVO" >
<jsp:directive.include file="../common/header.jsp" />
<input type="hidden" id="bId" value=""/>
<input type="hidden" id="tax" value=""/>
<div style="display:none">
    <div id="cartCheckoutHiddenDiv">
    
    <form:input path="orderData" type="text" id="orderData" name="orderData" value=""/>
            <table    border="0" class="checkoutTable">
                <tr><td  >Order is placed for
                </td></tr>
                
                <tr><td ><form:label path="branchAdd1" /></td>
                <td><form:label path="branchAdd2" /></td></tr>
                <tr><td >Total Bill amount is : <span id="totalBill"></span></td></tr>
                <tr>
    </div>
</div>
</form:form>
</body>
</html>
</jsp:root>

VO -

Can someone please tell me why i am not been able to populate updated orderVo values? Is it because of AJAX? I have even used SessionAttribute but of no use.