problem in switching between different live&recorded resources

hello, everyone here. i've started a project to switch different live&recorded ADs .f4v streams.  i use the Stream Class as a hub to switch different live and record streams and finally to produce a merged stream.  everything is OK when i freely switch recorded videos, and the client side can view the merged video. however, when i switch from the playing recorded ad stream to live stream and vice versa, the produced merged stream viewed at the client side is freezed at the switching point, and after several mininutes the merged stream return to normal state and the viewers can view the switched new stream. then what is the matter? thank you for your reply. my partial asc file is as follows:

var stream = Stream.get("mp4:mergedvid.f4v");

application.onAppStart = function()

{

      trace("the app is start...");

      if(stream)

      {

            stream.record("append");

      }

}

Client.prototype.doStreamSwitch = function(streamName)
{

     if(stream)
    {
            stream.flush();
            stream.play(false);
            if(isLivePublishing)      // the stream is recorded ad stream or live stream
            {
                stream.play("mp4:" + streamName);
            }
     else
     {
           stream.play("mp4:" + streamName, 0, -1);   //the record ad will played from start to end if no other live or record stream switch in.
     }
   }

}