K2 Rest service returns result intermittent

Scenario

To retrive the count of tasks assigned to current login user in sharepoint

Implementaion

using below code in sharepont content editor webpart

function GetMyTasksCount() {
$.ajax({
url: 'http://k2dev.abc.com/K2Services/REST.svc/Worklist/Items?$format=json&filterXml=<Criteria xmlns="http://schemas.k2.com/worklist/d1"><Filter Field="ProcessName" Comparison="Equal" ValueType="String">Test Wokflow</Filter></Criteria>',
method: 'GET',
crossDomain: false,
error: function (data, error, status) {
$("#MyTaskCount").empty();
$("#MyTaskCount").append("<p>Failed to retirve tasks</p>");
},
success: function (data) {
tasks = data;
ShowMyTasksCount();
}
});
}

Issue

sometimes it works and sometimes it returns result as unknown.