Marlin 81 feed problem
What you're experiencing is most likely due to a broken (or missing) cartridge guide spring. The guide spring is simply a flat spring located immediately above the chamber opening. You can easily see it when the bolt is open. The spring should protrude approximately 3/32" straight back. As you can probably envision, this spring makes contact with the nose of the bullet as it loads into the chamber. The spring guides the bullet nose into the chamber.
Replacement is a relatively simple procedure. GPC has the spring in stock (I am in the process of the same repair on one of my two 81s. The spring is about $1.35 plus S&H. The most difficult part of this repair is removing the barrel from the receiver. Remove the action from the stock. On the underside of the barrelled action you should see a cross pin holding the barrel into the receiver. Drive the pin out. Now depending on age and rust, the barrel can be removed from the action. I clamp the barrel into a bench vise using 1/4" thick red rubber to cusion the barrel and provide better grip. Then, with a brass punch and hammer, drive the action away from the barrel. If age/rust is a problem apply some Kroil to the area and allow to soak in. This may require any number of applications. I've never had to use the Kroil more than once, but have heard real nightmare stories. If necessary, use some heat from a propane or Mapp torch on the receiver. Do not over heat the material since that will compromise the properties of the steel. In addition you do not want to apply too much heat that it causes scale inside the barrel. Once the barrel is free and separated, you'll see where the cartridge guide spring sits in a shallow recess ont he top side of the barrel. Repalce it with the new one, clean any rust from the mating surfaces and reassemble.
Really not a big deal considering what a good shooters the old 81s are.
Good luck!
