the gut string did break..and you have a substitute c nylon (also 2nd oktave) In this case you cant make very much wrong. The nylon string sounding different than gut but after a day it becomes more clear. If you dont have used a 2nd octave c nylon…but a other one with different diameters…than there might be a problem.
Or, if the knot is not resting against the harp, if there is some complication in the knotting, that could perhaps interfere; or it could be a defective string, or not a good one. Being crisp and clear are just the advantages of nylon over gut, if you want that. It may also be the way you are playing it, nylon requiring a different touch than gut. Try playing that string more easily, less pressure and see if that helps any.