invoking ReadBytesAsync method sometimes never return

[yushu3166]

code:
if (!await ConnectAsync())
{
return null;
}
Stopwatch stopwatch = Stopwatch.StartNew();
stopwatch.Start();
try
{
var cts = new CancellationTokenSource(1000 * 3);
byte[] bytes = await _Plc.ReadBytesAsync(DataType.DataBlock, DB, start, length, cts.Token);
stopwatch.Stop();
if (bytes == null || bytes.Length == 0)
{
_LogUtil.Error($"S7Plc error:{stopwatch.Elapsed.TotalMilliseconds}，IP：{_Plc.IP}，DB：{DB}，start：{start}，length：{length},MaxPDUSize:{_Plc.MaxPDUSize}");
}
return bytes;

}
catch (Exception ex)
{
stopwatch.Stop();
_LogUtil.Error($"S7Plc Exception:{stopwatch.Elapsed.TotalMilliseconds}，IP：{_Plc.IP}，DB：{DB}，start：{start}，length：{length},MaxPDUSize:{_Plc.MaxPDUSize}, ex:{ex.Message}");
}
return null;

=============

byte[] bytes = await _Plc.ReadBytesAsync(DataType.DataBlock, DB, start, length, cts.Token); // sometimes here never return or throw exception,why?