filterColumn does not work in yajra/laravel-datatables

to show list of Users I use https://github.com/yajra/laravel-datatables

In addition to Datatables own search box to filter results, I have a combobox to filter user based on their level.

This is My Combobox :

<div class="form-group" id="levelFilterWrapper">
                                <label for="levelFilter">Userlevel :</label>
                                <select id="levelFilter" name="levelFilter" class="form-control">
                                    <option value="all">All</option>
                                    <option value="admin">admin</option>
                                    <option value="user">user</option>
                                    <option value="centerManager">centerAdmin</option>
                                </select>
                            </div>

And this is My jQuery Code :

var allUsersTable =
                $('#allUsersTable').DataTable({
                    processing: true,
                    serverSide: true,
                    "bSort": false,
                    "responsive": true,
                    ajax: {
                        url :   '{!! route('usersData') !!}',
                        data: function(d){
                            d.level =   $('#levelFilter').val()
                        }
                    },
                    columns: [
                        {data: 'checkbox', name: 'checkbox', "width": "20px"},
                        {data: 'userPic', name: 'userPic', 'className': 'text-center'},
                        {data: 'email', name: 'email'},
                        {data: 'created_at', name: 'created_at'},
                        {data: 'level', name: 'level', 'className': 'text-center'},
                        {data: 'actions', name: 'actions', 'className': 'text-center'}
                    ]
                });

        $('#levelFilter').change(function (e) {


            allUsersTable.draw();
        });

And This is My yajra Datatables laravel Code :

$users = User::select(['user_id', 'name', 'family', 'email', 'created_at', 'level']);

        $datatable = app('datatables')->of($users)
            ->filter(function ($query) use ($request) {
                if (!empty($request->get('level')) and $request->get('level') != 'all') {
                    return $query->whereLevel($request->get('level'));
                } else {
                    return $query;
                }
            })
            ->orderBy('created_at', 'desc')
            ->addColumn('checkbox', '<input type="checkbox" name="item_id[]" value="{{$user_id}}" id="Check_{{$user_id}}" class="minimal">')
            ->addColumn('userPic', function ($user) {
                return '<img src="http://www.gravatar.com/avatar/' . md5($user->email) . '?d=mm&s=30" class="img-thumbnail img-responsive">';
            })
            ->editColumn('email', function ($user) {
                return '
                        ' . $user->name . ' ' . $user->family . '
                        <div class="en-text"><a href="mailto:' . $user->email . '" target="_blank">' . $user->email . '</a></div>
                    ';
            })

            ->editColumn('created_at', function ($user) {
                return JDF::jdate('l j F Y - G:i', $user->created_at);
            })
            ->addColumn('actions', function ($user) {
                return '
                        <a href="/admin/user/' . $user->user_id . '/edit" class="btn btn-primary btn-sm btn-flat fa fa-pencil-square-o" data-toggle="tooltip" data-placement="top" title="Edit And View"></a>
                        <a href="#" class="btn btn-danger btn-sm btn-flat fa fa-trash-o delItem" data-toggle="tooltip" data-placement="top" title="Delete" data-item-id="' . $user->user_id . '"></a>
                    ';
            });

        if ($keyword = $request->get('search')['value']) {
            $datatable->filterColumn('name', 'where', '=', "%$keyword%");
        }

        return $datatable->make(true);

filtering via levelFilter combobox works fine But I want when Admin select a user level form that, filtering via DataTable search box , search in that selected level. But after type some characters in search box , again shows exactly same results from combobox filtering and seems search box filtering does not work in this case