The robot, called Dextre, flew in pieces to the station aboard the shuttle Endeavour. It has been assembled over the course of three spacewalks. So far, the 12-foot robot and both of its 11-foot arms have checked out fine.
On Tuesday evening, astronauts plan to use the space station's mechanical arm to attach Dextre to the outside of the station's U.S. lab, Destiny.
The spacewalkers' robot-related chores went smoothly, but Behnken had some trouble attaching a science experiment to Europe's Columbus lab. He couldn't get the suitcase-sized experiment to latch firmly onto the platform.
Behnken ended up carrying the experiment back to Endeavour's payload bay, repeatedly expressing his disappointment.
"You gave it your all, Bob," Linnehan said. "No one could have done any better."
Experts on the ground will spend the next couple of days trying to figure out what's causing the problem and how to fix it or work around it.
Astronauts participating in one of the mission's final two spacewalks may end up tying down the experiment and its twin instead of bolting them, said Zebulon Scoville, the lead spacewalk officer for Endeavour's mission.
Despite the problem, Scoville said he couldn't have been more pleased with the spacewalkers' work.
"You're making rock stars question their job choice," Scoville radioed Behnken before asking him several questions about the troublesome fitting.
Five spacewalks are planned for Endeavour's 16-day flight, which is about halfway done. While shuttle astronauts have performed five spacewalks before on a single flight -- on trips to the Hubble Space Telescope -- it will be a record for a shuttle-station mission.
In addition to delivering Dextre to the space station, Endeavour's crew dropped off a storage compartment for the Japanese lab that will fly up in May. The astronauts not involved with the spacewalks -- including Japanese astronaut Takao Doi -- continued setting up the storage compartment in preparation for the arrival of the lab, which is named Kibo. The word is Japanese for hope.