336s chambered in .44 Mag are notoriously unreliable and prone to feeding issues. The 'smith is not full of bull, if it's a 336.
As for "Marlin Jams"....
The 'classic' Marlin Jam is a cartridge on the carrier, being fed into the chamber, while another cartridge has been released and is
under the carrier. This results in total lockup of the rifle, inability to extract the upper round, inability to move the lower round, and requires disassembly to clear. This type of jam is exceedingly uncommon in post-1960 Marlins, since they went to redesigned carriers. (But it is encountered in the original .44 Mag 336s.)
The most common jam that is often also referred to as a 'Marlin Jam' is what Marlin calls "letting in two". You get a cartridge on the carrier, and the case head of another cartridge sticking out of the magazine far enough to interfere with the carrier. This is most commonly caused by an improperly fitted carrier, the wrong carrier, improper length ammunition, a bent carrier, a worn out finger lever, and/or a worn out carrier (the cam surface on the underside). However, it is occasionally caused by the wrong magazine spring and/or plunger, combined with a slightly worn or improperly fitted carrier.
This type of jam
usually results in total lockup of the action, but it can be cleared if you can push the second cartridge back into the magazine, through the loading gate. (Difficult, but possible with a pocket knife or screw driver.) It's best to disassemble the rifle to clear the jam.
But..... Snyper speaks the truth:
About three quarters of the time when a Marlin jams, it's a loose action screw
Loose loading gate screws are the #1 cause of feeding and cycling issues in 336s and 1894s.