Pitchers and coaches from both the Dodgers and Astros complained Saturday night about the World Series baseballs—and this time the controversy is not just about liveliness. They say there is a new problem: the baseballs used in the World Series are slicker than the ones used in the regular season because of a difference in the grain of the leather. The slicker World Series balls particularly make it hard to throw a good slider, they claim.

“We had a well-pitched game tonight from both sides,” Astros pitching coach Brent Strom said after Los Angeles won Game 4, 6-2. “I’m not taking anything away from the players. I just want to know why? Why in the world would the baseballs in the World Series be different? Because you can see the difference. You can feel it. I don’t understand it at all.”

Said Dodgers pitching coach Rick Honeycutt, referring to his Game 3 starter, Yu Darvish, “Yu noticed the difference. He told me the balls were slicker and he had trouble throwing the slider because of how slick they were. He wasn’t able to throw his slider the same way.”

Peter Woodfork, senior vice president, baseball operations, of Major League Baseball, said World Series balls are tested at the time of manufacturing and are made from the same materials and to the same standards as regular season baseballs. “The only difference is the gold stamping on the baseballs,” he said, a switch from the blue ink used during the regular season.

Major league baseballs are manufactured in Costa Rica, using cowhide for the cover. An MLB source speculated that how the baseballs were treated with the pregame rubbing mud before Game 4 could have led to a perceived problem.