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The Democratic political figure said he met Thursday with Maduro -- whom, unlike President Donald Trump's administration, he referred to as president -- and earlier spoke to him by telephone.
"I am glad we were able to meet with President Maduro to discuss the potential release of the American prisoners and other COVID-19 humanitarian issues," said Richardson, a former US ambassador to the United Nations and governor of New Mexico.
"We regret that we were not able to secure the release of the Americans," he said in a statement.
Richardson said he was seeking to free two US military veterans, Airan Berry and Luke Denman, who were detained amid a shady, failed sea attack on Venezuela in May.
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Officials said that a private US security company backed by Venezuelan dissidents carried out the mission in a purported bid to snatch leaders.
Richardson was also seeking to free six former executives of the oil company Citgo -- five of them dual US-Venezuela nationals and the other a US resident -- who were first arrested in November 2017.
Roger Carstens, the US envoy for hostage affairs, said last month that all six men were "in mortal danger," with several displaying symptoms in line with COVID-19.
Richardson has repeatedly undertaken missions with US adversaries such as Iran and North Korea to free Americans. A fluent Spanish speaker, he said he also met twice in Venezuela with Vice President Jorge Rodriguez.
Richardson's non-governmental efforts have drawn criticism from some hawkish Americans, although Richardson in his statement said had a "partnership" with officials in the Trump administration.
Trump has sought unsuccessfully since January 2019 to overthrow Maduro, a leftist who presides over a crumbling economy and whose re-election was widely seen as marred by irregularities.
sct/dw