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John -- More than one cause can exist. But all stem from lack of attention over time. The heart of that transmission is the sliding gear shaft. To shift into gear, one of two gears is moved along this shaft to close the link between reduction gear and the main (differential gear). A gear is moved by a shifter that falls into a detent on the shifter shaft to contain the gear in its engaged position. Two different shifters are involved when looking at 1st & 2nd gear so the only thing common when thinking shifters is shifter shaft adjustment -- one possible cause. The sliding gear shaft is borne on each end by a tapered (timken) bearing. These bearings are to be loaded so there is no end-play in the shaft yet the shaft is free to turn. Over time, the bearings wear which (in turn) allows for some slop in how the shaft is supported. Any slight skew or miss-alignment can place the gears at an angle when under a load and they will eventually migrate along the sliding shaft, overcoming the shifter shaft detent and disengage. How to fix is to basically overhaul the transmission. Depending on how many years the gears have been allowed to jump -- they too could have some wear pattern by now that contributes to the problem! This will involve a bit of a major teardown in terms of removals like the clutch, belt pulley, pulley dust shield, and gear cover on the right side; the top transmission cover and perhaps even the flywheel. Ideally, both rear wheels are off, axles removed along with the differential & rear transmission cover. Then you have the transmission in full view!
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