1- the string should be wrapped at the very bottom of the post for a lot of pressure to be placed on the nut.
2- if this doesn't work and it keeps popping out, file down your nut very carefully, just a little bit at a time.
3- check your action and truss rod to see if there is any case of a backbow. That's probably where the buzz is coming from. Also when doing so Make sure the action and truss rod accommodate one another.