Probably check out Wolf Gunsprings and see what the difference is in factory spring weights. You'll want the lighter spring for the 9mm or the gun may not function correctly with a heavier spring.
ETA, I just looked an it appears as if both the .40 and the 9mm use the same 22lbs spring. If you're having issues you may try the lighter spring for the 9mm.
Why the thumbs down?