I am NOT any kind of expert. With that out of the way, I'll share what I think might be the problem . . . the feed block is worn out. The older feed blocks were of a two piece design. With use causing wear, the two pieces separate just enough, to cause feeding problems. With the new one piece feed throat, you will need a new style lifter and the newer style lifter spring. The newer style lifter spring, also serves as the ejector.
There are a few things to know in advance. You will have to file (grind, carefully) one of the "nubs" off the new style feed throat. Depending on the style of bolt you have, you may have to take some material off the new feed throat. That's no big deal. When re-assembling the lower action into your rifle, 1) be sure the hammer is cocked and 2) be sure the bolt is in the locked back position.
As has been said, Rimfire Central has a ton of information about Marlin (Glenfield) Model 60s.