All good suggestions! I have also had the frustration of misfires in my .35Rem and 4T carbines and was once chastised for changing from the "perfected" Marlin springs to "Wolf" springs, which did not solve the problem, although it did improve the trigger functioning. I found two problems causing my misfires: one of which has been addressed - the Lee auto prime I was using had a worn out "toggle" and was not seating all the primers fully (immediately corrected), and the COAL of my loads were a "smidgen" too long, which cushioned the firing pin enough to cause a misfire. Even though the primer on a misfire would be indented, I could recycle them a second time and usually they would fire. This cushioning effect was mentioned on another site in regards to cast bullet loading where extra wide meplat bullets had to be seated deeper in the case to function properly. I discovered that the short throat of my carbines required I pay closer attention to limiting OAL to the maximum recommended by the bullet manufactures' to insure consistent ignition every time!
I also switched to Federal 210 primers, which is also recommended for "drillings" and antique arms shooters because of their softer cups and more sensitive priming compound. My 444P's throat is about .10 shorter than the 444SS according to most sources - I now believe it to be true. The easy "breakdown" of the Marlin for cleaning sure makes troubleshooting the firing mechanism a lot easier than any other lever actions made if "crude" is part of your problem! Good luck with yours.