Why sikuli cannot capture the scrrenshots if a game at its full screen mode

I want to use sikuli to automate some games operations, I found if the game is in the windowed mode, sikuli can work normally, but if I make the game at full screen mode, sikuli always run with the error "Find failed: cannot find xxxxx.png".
Can anyone help give me a explanation? does anyone meet the same problem? or sikuli cannot support games because of the games display mechanism？