Make sure there are not any unnecessary twists or bends in the line. If thats not the issue I would suspect that the cable inside the housing itself is damaged and is allowing the cable to travel smoothly only in one direction. It may have difficulty retracting due to the broken fibers in the cable. In other words broken fibers would cause the cable to move against the grain of the housing.
Sticky throttle is dangerous, dont mess with it, just replace it.