I'd look at the cartridge stops.
This is a somewhat common problem on Mod31's that have seen some use.
Some drop the shell as a loaded round is chambered/action closed.
Some drop a round when the gun is fired (recoil)
The 31 uses 2 cart stops.
IIRC it's the Left side cart stop that gives the trouble.
It lets a round skip by as the action is closed and the round just dumps out onto the ground.
(Might have my L & R mixed up,,and when you look at the thing upside down remember that it's reversed again).
The Right side cart stop holds the shells back inside the mag while the feeding and chambering is going on.
Then when the bolt is closed and locked,,that Right side Cart Stop releases and allows the ammo in the tube to run forward only to be caught by the Left side cart stop.
There it holds that first round ready for the carrier to accept it when the action is pumped again.
If the edge of that Left side stop is worn or otherwise not catching the rim of the hull, that round skips by the Stop and lands on the ground.
This all takes place in a moment as you are chambering the fresh round in the chamber,,,one in the chamber,,one onto the ground from the magazine.
You can shop around and usually buy a replacement cartridge stop.
-They are gauge specific.
The Model 31 made some changes in some internal parts as the mfg'g went along. I don't know if they changed the cartridge stops at all.
-You will most likely get a worn part from a 'parts guy' as that is where they come from now. Worn out guns stripped for parts.
-You can take the cart stop out and carefully bend the tip outward just a few .000 to give it a better reach into the mag. That will usually fix the problem just as well.
I've seen them built up on the tip w/ weld and reshaped. That seems like over-kill to me when a simple bend will do.
Unless you like to demonstrate welding skills, or the part tip is broken off all together, I'd just put a bend in it.
Hulls that have a rim dia just a few .000 smaller in dia can also show the problem.
Especially in a gun that has some wear in it as above. But perhaps not enough when a different brand is used.
Hope this helps.