"Falling lever syndrome" is common with Marlins, and the problem (and solution) lies in the lever plunger, the little "latch" that holds the lever shut. Usually, either the edge of the plunger, which should be crisp, is a bit rounded, or the spring backing the plunger has grown loose, or there is an accumulation of gunk inside the hole the plunger rides in, restricting its travel.
The sure fix, requiring no further diagnosis, is to replace plunger and spring. This works so well, in fact, you may find the lever is now locked too tightly -- no problem, just round the edge a tiny bit with a fine file.
But before you go to that length, you can remove the plunger and spring, clean out the hole, and maybe stretch the spring a bit. Or, you can use a fine file and re-establish the edge of the plunger end (this is what Marlin told me, years ago, they did to fix 90% of the guns with this problem). Or, you might find that just a spring replacement will do the job, or maybe just a plunger replacement.
Any way you go, the repair is easy and inexpensive.
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