What Model Marlin is it?
Here is what I would try. Make sure that the firing pin and the area inside the bolt that holds the firing pin is clean. Then slightly stretch the firing pin spring and reassemble. What ever you do do NOT file the firing pin to make it sharper ( not saying that you would try that but I have seen a many persons do that to .22 firing pins thinking it would help.) The pins are case hardened and once you remove the hardened metal they are useless.
By posting up the Model of the firearm we may be able to help further