On some rimfires, if they are dry fired it puts a dent in the barrel from the firing pin. Once the round is fired and the brass expands, the burr created in the back of the chamber can be enough to cause hard extraction. This is true of revolvers, semi autos, and any other rimfire. The easy fix is to run a chamber reamer in to trim off the burr. This can often cure other chamber problems at the same time.