You might have a busted spring somewhere inside, or something up with the detent interlock mechanism, but id check simplest first.

Its possible the allen head bolt holding the reverse shift arm is loose or binding and its not going all the way into gear. Since reverse has its own shift arm, an issue with the shift arm would affect nothing else, whereas something wrong inside might affect more than reverse.

Bottom line the gearbox isnt terrible to take apart, you might have to do that

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