can not get a list<> using Jquery.ajax

[webmethod]
[ScriptMethod(ResponseFormat=ResponseFormat.Json)]
 public static List<Problem> GetProblems()
 {
     List<Problem> allproblems = new List<Problem>();
     using (TMEntities tm = new TMEntities())
     {
         allproblems = tm.Problems.ToList();
     }
    return allproblems;
 }

<script type="text/javascript">
       $(document).ready(function () {
              $.ajax({
                   type:"POST",
                   url: "WebForm1.aspx/GetProblems",
                   data: "{}",
                   datatype: "json",
                   contentType: "application/json; charset=utf-8",
                   success: function (data) {
                       alert('success');
                   },
                   failure: function (response) {
                       alert("fail");
                   }
               });
           });
   </script>