You may have solved your problem by now, but here is a quote I just read on another forum about misfires with the Marlin 39A.
"Misfires were already something I knew about, on rebounding hammer Winchesters, too, an easy fix. The hammer strut on rebounders has a forked end, the upper fork powering the hammer forward, the lower fork striking the hammer base and causing it to rebound to half-cock, the mainspring fighting itself. Simply shortening the lower fork will cure misfires, but might not allow the hammer to rebound to half-cock (if that's important to you). Only tools required being the screwdriver for the buttstock tang screw, and files. The "hammer spring adjusting plate" aka the strut seat easily pops out and back in using only hands, pivoting out the top end first, then sliding the lower end out of the milled lower tang slot, reassembly the reverse."
