I've found that it helps when re-stringing, to start (after putting the string through the hole), by taking the FIRST wrap around the post and go OVER the string through the hole. Then, on the second wrap around (and the consecutive times) go under the string through the hole so as to "pinch" the string as it continues to tighten. I've found this helps especially with the plain steel strings. Hope that helps.

If this is still a problem a reliable, local music store should be able to help you with your query.

Good luck!!