I had a similar problem 4 years ago, the plunger broke, had it repaired, about a year later, after shooting a lot of dove and quail, it broke again. I sent to Benelli for repair. They replaced the whole trigger assembly and a couple of springs. They gave no reason for the problem except to say the trigger assembly was “worn out”. That was three years ago. It just broke again. The gun is over 20 years old, I bought it used in 1998. Trying to decide if I want to keep replacing plungers or trash it.
