Take the crank off the shaft, now with your hands, feel if there is any **play** in the threads.
Wiggle it up and down, not in and out.
If there is any play, Loctite will not be able to bridge the gap in the threads to hold it.
Those cranks could be 50 years old, so it's very conceivable that the threads are blown out.
Time to call PBR.