Is the shift linkage out of adjustment? Unhook the shift cable at the outboard and see if you can manually shift it in reverse. if so take note of the position the linkage needs to be to engage reverse hook it back up and see if the travel is far enough to engage revers. If not adjust shift cable. (small increment) Confirm adjuster is working correctly. Consider that I am not a mechanic but have had a similar issue when considering my advice. A bent shift pin in the leg can also cause this. Or RPM not dropping low enough at idle for shift dog to engage.
