@HttpPost
    global static void doPost(){
        //Set up for XML processing
       RestRequest req = RestContext.request;
        RestResponse res = RestContext.response;
        string strMsg = req.requestBody.toString();  
        System.debug('Request Contents' + strMsg);
        XmlStreamReader reader = new XmlStreamReader(strMsg);
        //Deserialize
        while(reader.hasNext()){
            if(reader.getLocalName() == 'ToUserName'){
                reader.next();
                if(String.isNotBlank(reader.getText())){
                    toUserName = reader.getText();
                }
            }
            else if(reader.getLocalName() == 'FromUserName'){
                reader.next();
                if(String.isNotBlank(reader.getText())){
                    fromUserName = reader.getText();
                }
            }
            else if(reader.getLocalName() == 'MsgType'){
                reader.next();
                if(String.isNotBlank(reader.getText())){
                    msgType = reader.getText();
                }
            }
            reader.next();
        }
    }

Test Method:
static testMethod void testDoPost(){
        RestRequest req = new RestRequest();
        RestResponse res = new RestResponse();
        String strTmp = '<xml><ToUserName>tUN</ToUserName><FromUserName>fUN</FromUserName><MsgType>mT</MsgType></xml>';
        req.requestBody = Blob.valueOf(strTmp);
        req.requestURI = 'https://ap1.salesforce.com/services/apexrest/Demo//CoreService';
        req.httpMethod = 'POST';
        test.startTest();
        RestContext.request = req;
        RestContext.response = res;
        ApexRESTCycle.doPost();
        XmlStreamReader reader = new XmlStreamReader(req.requestBody.toString());
        test.stopTest();
        System.assert(ApexRESTCycleExample.fromUName == 'fUN');
        System.assert(ApexRESTCycleExample.toUName == 'tUN');
        System.assert(ApexRESTCycleExample.mType == 'mT');
}

The issue now is in the Post method, only the first node <ToUserName> got covered, other nodes are not covered, error attached on above.

I need a perfect coverage, please help.

Thanks,
Helen