Irregular Cocking
The usual culprit is a trigger has got some dirt or crud in it, not allowing the cocking piece to engage the cocking bar atop the trigger. Sometimes crud can build up between the top of the trigger and the action body, not allowing the cocking bar to come all the way up.
That, or someone has done some adjustments that are out of whack, such as the sear engagement, or got too light on the return spring.
Get some spray brake cleaner and clean the thing real good, using the little red tube to get down it there real good. Then wash it out with lighter fluid. That might fix the problem...........jackie