No way for me to know, could be any number of things up to and including how kmplayer decides to process key strokes. Hard to say, honestly. Definitely make sure it's activated (the window), otherwise you could try using Spy++ to watch the messages sent when you physically press the keys versus when S+ sends them.
You could also try using acSendShiftDown(), then acSendKeys("{PLUS}") then acSendShiftUp(). Some apps are finicky about the instantaneous nature of receiving the keystrokes. Also try adding some delays between the commands I just mentioned (acDelay(20) or something).
how can i watch the messages sent with spy++ is there a tutorial. i am confused if winspy++(the link you gave in the other post) and spy++ are different :| searching google for winspy++ brought only hacking tutorials :|
Spy++ comes with Visual Studio, it's what I use. I only know of WinSpy from hearing about it. You can download Visual Studio Community and check the SDK/Tools option (I don't remember which) and Spy++ should be installed as well. Of course, Visual Studio is a huge thing to download/install for just one app. There are probably other message watching apps.