5th fret harmonics are really tough. They require the most precise touch of all the harmonics.
1 - make sure your finger is directly over the fret.
2 - as soon as you pick the string, pull your left hand finger off the string.
I will say that some banjos make 5th string harmonics easier than others. I’ve played on some cheaper banjos that were almost impossible to get a ring out of the 5th fret.