A screen may not be updated at the time of the return from system task bar

[hanada@basil.freemail.ne.jp]

I am using Real VNC-V4.1.1 on windows platform. When I minimize Real VNC
and maximize it from "system task bar", it does not update the screen from
time to time.
It seems that Real VNC server does not send a new data to the client.
Because the screen is updated when I perform "Refresh Screen" by F8 key.
Is this Real VNC's bug?
Does anyone have the same problem?

Thanks,
Hiromitsu.