Bigcting

My son just finished the roller coaster, the only problem is that the chain doesn’t move smoothly. When I crank the links at the the top between the tires (counter clockwise) everything runs in the right direction, it’s just very difficult to move. I have seen video and I know that Lego makes sure there sets work, but for the life of me, I can’t figure out what’s wrong. Any help suggestions would be appreciated.