Welcome to forum Tuffel, I would first check shifter hi low cable to ensure no excessive slack. If that looks ok then it may be the shifter fork (internal mechanism that slide gear to another position on its shaft) is bent or leading edge of engaging dog teeth are rounded or worn on the gear that engages or disengage high/low range. I’m only suggesting from experience on garden tractor transmission that kept popping out of gear. I hope you’re issue is cable slack which would be easy fix.
