I've had that very same thing happen ..... When cocking the rifle the lever slipped out my fingers ... Slammed back and refused to budge
Opened it up and the top sear had snapped .... I think by the way , and where it had cracked it was at a weak point .... Perhaps a deliberat weak point .... To save further damage ???
Anyway i promptly donored the same sear out of my 400 and have been careful ever since
Hate to say it , but in general I think the gun is way over priced