First check for carbon and/or any goo buildup on the extractor, and give it a good cleaning. Did you clean and lube the gun before trying it out ? Then look for any what might appear to be any excessive wear on the extractor. If the extractor looks good, I'd suggest some new springs from Wolff.
Look on this page
http://www.gunsprings.com/SemiAuto/SmithWessonNF.html#Numbered
for this section
EXTRACTOR SPRINGS - Extra Power
Extractor spring - 6.5 pounds (factory spring is 6 pounds)
For Use in:
39, 439, 539, 3904, 3906, 3913,3914,3944, 3946, 3953, 3954,
59, 459, 559, 659, 5903, 5904, 5906, 5923, 5924, 5926, 5943,
5944, 5946, 5967,
410, 411, 4003,4004, 4006, 4026, 4043, 4044, 4046,
910, 915
Stock No. 27125 - each.....$ 3.49