Good advice from the folks here. One more little thing: if you find it IS the nut doing what Ragman says, be careful 'widening' the slot. Too wide and it isn't good. Something I have found that helps make the srings of my six-string slide more easily over (thru) the nut in graphite. The liquid stuff used by people who have guns works as well as the old sharpened pencil did. BUT, it can be messy if you're not careful. It doesn't take much. You can use an old string (the one you're replacing) to get the stuff in there.