Spies Fail Lie Detector Tests?

How do the CIA, FBI and US Army explain the long periods Alexander Yuk Ching Ma, Jerry Chun Shing Lee and Peter Rafael Dzibinski Debbins spied for China and Russia without being detected? Mr. Ma was employed by the CIA for 22 years and the FBI for six. For nine years, Mr. Lee first worked for the CIA and then the FBI, and Mr Debbins spent his nine years in chemical and Special Forces units within the US Army. Each man provided highly classified national defense material to US enemies. Mr. Ma also compromised US human assets in China, and Mr. Debbins recommended fellow Green Berets who might cooperate with Russian intelligence. Periodic lie detector tests, identifying suspicious coincidences, "walking back the cat" investigations to discover those who had access to information about a failed operation, security clearance questionnaires and other procedures for uncovering spies exist. But Robert Hanssen, an FBI counterintelligence analyst, never had a polygraph test during the 15 years he spied for Moscow. Another spy revealed he had been able to convince a new lie detector operator results that indicated he was lying were inconclusive. What methods should prevent US spies from escaping detection before they are able to undermine US national security?