Mirror lock can be caused by a dozen different things. First being wrong batteries, dead batteries. A constant battery check LED is first step. Blinking means batteries are wrong or weak. Silver oxide 1.55v are the only correct ones. 357 , MS76, KS76, SR44-W, G-13, S76, 303. B or battery check/reset will reset the mirror. Sometimes the magnet is bad or "open" , usually from tampering, this rips the magnet winding. Bad circuit, or misaligned, mis adjusted timing levers. Dirty magnet can cause this, thought it would be erratic and not happen every time.
John