The search bar does not show any results

I’m having a problem with the search bar. I typed the text I wanted to search into the search box but found nothing appeared even though the text was displayed in the normal ListView builder. Before I tried refactoring the code because I was stuck with the search bar. It doesn’t work (not showing search results), but my ListView builder works fine.
enter image description here

class _view_problemState extends State<view_problem> {
  TextEditingController _searchController = TextEditingController();

  List<problemModel> problemlist = [];
  List<problemModel> originalList = [];
  StreamController _streamController = StreamController();
  Future getAllProblem() async {
    problemlist = await problemcontrollers().getProblem();
    originalList = problemlist;
    _streamController.sink.add(problemlist);
  }

  @override
  void initState() {
    getAllProblem();
    super.initState();
  }

  void _filterSearch(String query) {
    List<problemModel> filteredList = [];
    for (var item in originalList) {
      if (item.toString().contains(query.toString())) {
        filteredList.add(item);
      }
    }
      setState(() {
      problemlist = filteredList;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Color.fromARGB(255, 14, 12, 134),
        title: const Text('search'),
      ),
      body: Container(
        child: Column(
          children: <Widget>[
            Padding(
              padding: const EdgeInsets.all(8.0),
              child: TextField(
                onChanged: (value) {
                  _filterSearch(value);
                },
                 controller: _searchController,
                decoration: InputDecoration(
                    labelText: "search",
                    prefixIcon: Icon(Icons.search),
                    border: OutlineInputBorder(
                    borderRadius: BorderRadius.all(Radius.circular(25.0)))),
              ),
            ),
            Expanded(
              child: StreamBuilder(
                stream: _streamController.stream,
                builder: (context, snapshots) {
                  if (snapshots.hasData) {
                    return ListView.builder(
                        itemCount: problemlist.length,
                        itemBuilder: ((context, index) {
                          problemModel problem = problemlist[index];
                          return Card(
                            margin: EdgeInsets.all(10),
                            child: ListTile(
                              title: Text(
                                problem.name_surname,
                                style: TextStyle(
                                    fontWeight: FontWeight.bold, fontSize: 19),
                              ),
                              subtitle: Text(
                                problem.area,
                                style: TextStyle(
                                    fontWeight: FontWeight.bold, fontSize: 17),
                              ),
                            ),
                          );
                        }));
                  }
                  return Center(
                    child: CircularProgressIndicator(),
                  );
                },
             ),
           )
         ],
       ),
     ),
   );
 }

}

>Solution :

  void _filterSearch(String query) {
    List<problemModel> filteredList = [];
    for (var item in originalList) {
      if (item.name_surname.toLowerCase().contains(query.toString())) {
        filteredList.add(item);
      }
    }
      setState(() {
      problemlist = filteredList;
    });
  }