About NetStream.step backward frame not work?

I created a stream connecting to FMS4.5 and  is able to Smart Seek with setting inBufferSeek=true,

detail's as below:

ns=new NetStream(nc);
ns.inBufferSeek=true;
ns.backBufferTime=60;
ns.bufferTime=3;
ns.play("mp4:abc.f4v");

it's ok to pause at 5's while playing fine.

when i want to backward frame i use

ns.step(-1); //work
it works.

however, when i firstly pause the stream and then seek to any time point, eg:

ns.pause();
...
...
ns.seek(20);

After recieving NetStream.Seek.Complete, when i tried to backward/forward frames, it does not work at all. eg:

ns.step(-1); //not work
ns.step(1); //not work

If i run ns.resume(); ns.pause(); it was able to forward frames but not backwrad.  eg:

ns.pause();
...
...
ns.seek(20);
...
...
if(e.info.code=="NetStream.Seek.Complete"){
   ns.resume();
   ns.pause();
}
...
...
ns.step(-1); //not work
ns.step(1); //work

So my question is, how to make it work to backward frames after that seeking has completed ?
or, is there any possibility that this issue could be a bug with flashplayer or FMS?

Thx.