the screen will be black when click the screenshot

Dear, All
       I have install a sikuli on our demo machine yesterday, which have two display cards.
       - the OS is linux-SUSE,
       - the version installed named "Sikuli-X-1.0rc3 (r905)-linux-x86_64".
       The issue is that:
       the screen will turn into BLACK@@ , when I click the "Screenshot". So I can not capture any picture in the demo machine. All of them are black.
       I am sorry that my English is not very good @@

       Does anybody know what cause that black screen problem?

Best Regards,
XiHao