Google Drive API - Search by file name only finds the file if it has recently been opened via Google Drive on chrome

I have a method that will search for a file by name and then download that file:

    public static void DownloadFile(string filename)
    {
        service = GetDriveService();

        //check if file exists and grab id 
        FilesResource.ListRequest listRequest = service.Files.List();
        listRequest.SupportsAllDrives = true;
        listRequest.IncludeItemsFromAllDrives = true;
        listRequest.PageSize = 1000;
        listRequest.Q = "name = '" + filename + ".pdf'";
        FileList files = listRequest.Execute();

        if (files.Files.Count > 0) //the file exists, DOWNLOAD
        {
            var request = service.Files.Get(files.Files[0].Id);
            var stream = new System.IO.MemoryStream();
            request.Download(stream);
        }
    }

The problem is that FileList files = listRequest.Execute(); will return 0 files unless I have recently opened that file on Google Drive in Chrome. it's like the files are not indexed. Am I missing a parameter?

I've tested this on loads of files and without fail, if I've opened the file on chrome FileList files = listRequest.Execute(); returns 1 element otherwise it returns nothing.

I have also tested searching in specific folders only, same issue.

Any help is much appreciated.

thanks

edit; Here is the GetDriveService Method:

 private static DriveService GetDriveService()
    {
        string[] scopes = new string[] { DriveService.Scope.Drive }; // Full access

        GoogleDrive cr = JsonConvert.DeserializeObject<GoogleDrive>(System.IO.File.ReadAllText(@"\\PATH_TO_JSONFILE\GoogleAPI.json")); 
                                                                                                                                                                                  
        ServiceAccountCredential xCred = new ServiceAccountCredential(new ServiceAccountCredential.Initializer(cr.client_email)
        {
            User = "xxxxx@xxxx.xx",
            Scopes = new[] { DriveService.Scope.Drive }
        }.FromPrivateKey(cr.private_key));

        DriveService service = new DriveService(new BaseClientService.Initializer()
        {
            HttpClientInitializer = xCred,
            ApplicationName = "APPLICATION_NAME",
        });
        return service;
    }

The User email is the same account I use to open the files in Chrome.