On your thinner strings, always do a non-slip back-twist of your string on the post.
I hope this quick pik explains it. So quick, when stringing, and the string will NEVER slip on the post again!

Place tag end of string thru hole in tuning post, pull tag end back around post and under string entry point on post. Pull tag end up, and tighten string. As the string tightens, the more pressure it puts on the underwrapped piece, ensuring no slippage.
IMHO ?