There could be a few causes.
The Marlin has a two piece firing pin so that the gun can not be fired until the gun is locked up. If the two firing pins are not aligned you will get misfires. Check if something preventing the two pieces from lining up.
The firing pin spring may be broken or weak. If it is broken it may prevent the rear firing pin from aligning properly
The firing pin itself may be broken. Many times when a firing pin breaks it will still impact the primer but not hard enough to ignite it.