Why can a nut be too tight?
The luthier delivers the already strung instrument and adjusts the nuts on bridge and saddle to the strings he has fitted. Steel core strings always have a smaller diameter than synthetic core or gut strings. If you now change strings with a smaller diameter to strings with a larger diameter, the string might now get jammed in these nuts and grooves.
Production line instruments are normally equipped with very thin “placemarking” strings (similar to a wire), so that the bridge and sound post stay in position. For this reason, the grooves and nuts are very tight. When changing strings, you must make sure that these grooves and nuts are adapted to the new strings by your luthier.