Main culprits:
Burrs on the edge - usually can be felt with a fingernail, or you can see changes in reflection by shining a light on it. A light - and I do mean light - wipe on a stone should help. Lay the inside faces flat, and don't remove any more than just the burr.
Grit/gunk wedged between the mating faces - clean well, in warm soapy water, and reoil when dry. Might not work immediately, but regular oiling should flush it out. An ultrasonic cleaner might help too.
Tired spring - the best fix for this is replacement of the spring.