UN and war crimes in Syria

How is it that the UN can 1) quickly determine whether war crimes are committed or not in a place when it takes it years to decide if Israel committed war crimes or not; 2) how come the UN always conveniently find quick evidence of war crimes in Syria but only by groups (regime and ISIS which are not on good terms with the US).  So Nusrah and FSA, which have done more sectarian kidnappings of Lebanese are blameless here?