Cable isn't connected, or you connected something wrong when installing it.. it's gotta be.
Usually there's what.. 2 shift cables?? Make sure they're all on the right levers, proper routing, etc.
Also... where are the detent balls on this trans? On the shifter or actually on the forks in the trans? Might've somehow dropped one out and that rolled away, that'd cause popping out of gear.
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