A few have had this problem, including me. The extractor and spring are the same as an AR. the original spring that comes with the WK has been found to be weak by some.
If you got the part kits there should be a little rubber o-ring that you can put around the extractor spring. This has solved the problem for some people. I decided to replace the entire extractor and spring and added the o-ring and an internal rubber plug that goes inside the spring (you can get them all from True North Arms). Didn't cost much and solved the problem.