The docking problem was the third such incident at the station in four months The Soyuz capsule carrying a US astronaut and two Russian cosmonauts has undocked from the International Space Station at the second attempt.
America’s Tracy Caldwell-Dyson and Russia’s Alexander Skvortsov and Mikhail Kornienko are due to land in Kazakhstan within hours.
The crew had been due to begin their descent to Earth on Friday, but the Russian spacecraft failed to undock.
The astronauts have been aboard the space station for almost six months.
Their first attempt to leave the station in the Soyuz capsule was aborted when computers signalled a technical problem with the hermetic seals.
Russia’s space agency said checks showed the station was sealed and it is unclear why a false signal was sent out.
Nasa said the crew had found a small piece of damaged equipment and experts were trying to work out whether this was causing the problem.
Friday’s incident was the third docking problem at the station in four months since the automatic system failed on two unmanned Russian Progress supply shuttles, causing one to fly past the station in June.
Nasa will be watching developments particularly closely because next year its space shuttle will be taken out of service, reports the BBC’s Richard Galpin from Moscow.
The only way to get Nasa astronauts to the International Space Station and back will be on board Russian Soyuz capsules, our correspondent says.
Russian cosmonaut Fyodor Yurchikhin and two Nasa astronauts, Doug Wheelock and Shannon Walker, are due to remain on the space station.
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