Usually going smaller strings isn't a problem on the nut, you sometimes have to file open the slots a bit for a larger string. You might have to adjust the truss rod changing sizes though or adjust bridge height etc. if your guitar has such.
If the sound is there on an open string, it is either the bridge or the nut. If you haven't been doing so, I'd try either some graphite or a bit of naval jelly in the string slots on the nut and bridge. That way the string isn't getting caught up on either when it stretches back and forth.
If the note is dying on a fretted point, you could have one of the frets that has come loose and is a bit high or is worn at a point causing some buzz.
― earlnash, Saturday, 7 April 2018 02:20 (six years ago) link