what he said, ditto
had the same problem with my newest Remington, and my old 1851 Navy-
file the notch deeper on the retaining latch groove
that's the little thing that hangs down under the front of barrel, and when you press the spring loaded latch in on the loading lever, it catches in the groove in that round part under the barrel
that notch that the latch catches into- take the thin side of a file, and grind it deeper so the spring in the loading lever, can fully push the latch into it
that fixed my 1851, but my newest 1858 still drops the lever
another thing you can do, is put a stronger spring in the loading lever latch assembly