Why can the bodies of the United States prisoners Russia take years of rear channel negotiations?

Another American detained in Russia was released as part of the negotiations to liberate it and two other Belarus, according to the White House. This comes after the release of American teacher Mark Vogel, who landed on the American soil after he was in a Russian prison for approximately four years for his marijuana. These negotiations are a diplomatic tight rope, which can often take years of back channel conversations. Roger Carrests, the chief negotiator of the hostage in President Biden, joins an explanation.