Kind of late to this discussion, but found it while researching a similar problem. I've got a Benelli m2 20 ga. and have lost several bolt handles.
I finally noticed that there was a bright mark in the receiver where the bolt handle was striking the inside of the receiver and was then forced out. Problem was that I was inserting the bolt handle in too far and it was protruding out the far side of the bolt, allowing it to hit the receiver and be jarred loose.
Have a look and see if you have a similar problem.