If you have the "crash box" 4 speed with the reverse lockout lever on the shifter, those transmissions are pretty flimsy, and are subject to wear. Chances are the shift rails in the top cover are the problem. I'd suggest removing the cover and disassembling the shift rods, looking for wear and/or broken parts such as springs and detent balls. It's also possible for the bearings and shift collars in the main gear set to get worn and hang up while they are being moved in and out of mesh. That box has no synchronizers to give trouble, which can be a good thing!
Jerry

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