Passing array of object from Ajax function to web service

I have two arrays of same object type that I am passing to a web service API. I look at the array just before it is passed to web service and each has an object in it. I put a breakpoint in API and the list is empty. Can't figure out what I am doing wrong.

I defined a DTO as:

public class AdminDTO
{
    public string fn { get; set; }
    public string mi { get; set; }
    public string ln { get; set; }
    public string email { get; set; }
    public string userid{ get; set; }
}

in JS function I populate two arrays as such:

// this is done inside a foreach loop
var fn = userData.FirstName;
var mi = userData.Initial;
var ln = userData.LastName;
var email = userData.Email;
var userid = userData.UserID;

//push the object onto the array
admins.push({
    "fn": fn,
    "mi": mi,
    "ln": ln,
    "email": email,
    "userid": userid
});
...
...
// clone admin list for later comparison
currAdmins = admins.slice(0);

At some point "admins" is changed through user interaction. Some objects are removed and some new ones are added. I use below to get two new arrays to hold the differences between "admins" and "currAdmins":

var adminsChanged = $(admins).not(currAdmins).length > 0 || $(currAdmins).not(admins).length > 0;
var adminsToAdd = [];
var adminsToRemove = [];

if (adminsChanged) {
    adminsToAdd = $(admins).not(currAdmins);
    adminsToRemove = $(currAdmins).not(admins);
}
...
// "newMapping" is a mapping of another DTO called "projctDTO", this is passed correctly to web service
doUpdate(newMapping, "projDTO", adminsToRemove, adminsToAdd);;

Now, I am trying to pass these two arrays to web serive via ajax call. They both have objects in them:

function doUpdate(mapping, jsonObjName, adminsToAdd, adminsToRemove) {debugger
    $.ajax({
        url: "../WebService/Blah.asmx/updateSomething",
        type: "POST",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        data: "{" + jsonObjName + ":" + JSON.stringify(mapping) + ", adminRemoveDTO: " + JSON.stringify(adminsToRemove) + ", adminAddDTO:" + JSON.stringify(adminsToAdd) + " }",
    }).done(function (result) {debugger
    ...
    });

Web service API:

public string UpdateSomething(ProjectDTO projDTO, List<AdminDTO> adminRemoveDTO, List<AdminDTO> adminAddDTO)
{
    // projDTO has stuff in it but both lists are empty
    ....
}

I also tried calling ajax function and setting ajax data like this, but didn't work:

doUpdate(newMapping, "projDTO", adminsToRemove, "adminRemoveDTO", adminsToAdd, "adminAddDTO");
...

function doUpdate(mapping, jsonObjName, adminsToAdd, jsonObjNameAR, adminsToRemove, jsonObjNameAA) {debugger
    $.ajax({
        url: "../WebService/AIR.asmx/updateProject",
        type: "POST",
        contentType: "application/json; charset=utf-8",
        dataType: "json",

        data: "{" + jsonObjName + ":" + JSON.stringify(mapping) + ", " + jsonObjNameAR + ":" + JSON.stringify(adminsToRemove) + ", " + jsonObjNameAA + ":" + JSON.stringify(adminsToAdd) + " }",