AutoIT on Virtual Machine doesn't work

Hello All! I have tried the same script on two computers (Win Serv 2008 both)
in RDP-session. On the one of them script works, on the another doesn't work!
Command "Run" works and my application starts
but then mouse doesn't move and doesn't make LeftClick!
Both Server 2008 are virtual machines.
  Please, give me a hint, why ONE OF THEM doesn't work?
Then I have tried my script on the real mashines with Windows-7 (without RDP).
On some mashines it works, but on some - doesn't work.
I changed speed of mouse's moving from 0 to 100 - no difference!
(No, there was a difference on the one virtual Server-2008: when
speed is less then 100 script did not move a mouse.)
  Only under XP my script works good (in RDP-session works too!
On a virtual machines under XP script works!)
My script is very simple:
  Run("...")
  $hWnd = WinWaitActive("...", 10)
  $size = WinGetPos($hWnd);
  MouseMove($size[0]+100, $size[1]+100, 100)
  MouseClick("left")
Last two commands on some machines doesn't work! I was looking
through FAQ and forum but did not find anything about this subject.
I tried ControlClick instead of MouseClick - no difference (under
  XP works, under virtual Server-2008 doesn't work).
I use only the last version of AutoIT on all machines.
Please help!
What should I do to force my script works?
  Thanks.