It sounds like one or two things need to be done.
First, make sure the bolt is clean and not gummed up with old dried oil and gun powder. Disassemble and clean the bolt.
The other thing to look at when you have the bolt apart is the firing pin. It's not really a "pin", it's made of flat stock. The lower back part of the firing pin is also a sear that is held back by the trigger until the trigger is pulled. I find a lot of these that have been filed down by a previous owner hoping to improve the trigger pull.
Have a look at this page and compare your firing pin to the #R117 to tell if it is worn out or had been filed down by someone.
This pic shows the relationship between the trigger sear and the firing pin sear. NOTE: this is not a 42M, just used to as an example.
A new firing pin can be found at Havlin Sales in MO, the best place to find old Mossberg parts. Or Wiseners might have one if Havlin doesn't.
Models 42B, 42C, 42M, 42M(a), 42M(b), 42MB, 42M(c), 42M(c), 42TR
Firing pin $18
This might help.
Also make sure that someone didn't over adjust the trigger adjustment screw (Part # R158) in the pic above. (see instruction sheet below)