I wonder if there was a feathering solenoid issue whereby the prop continually cycled between being feathered and unfeathered. If I remember correctly, the trick is to manually pull the button once the blades reach the feathered position. It's supposed to pop out by itself once oil pressure builds up in the feathering pump but the system's been known to fail.
It's been a long time since I worked on these aircraft so I could be wrong.