I am building a 21" scale cbg using the zero fret method for the first time. So last night, I finally get to the point putting one string (old guitar string I found while cleaning up the shop) on it to test tone and troubleshoot any problems. There was a little bit of buzz noise when I played it open. Then I tried to play the note in the first fret and it just "dank plunk dank" buzz sound. not even a musical note. same with second fret. The third fret is a bit cleaner and then down the fretboard from there it is fine.
The frets on this git are narrow low (like mandolin/banjo style). The zero fret is medium/medium. The frets appear to be seated properly. I am open to any suggestions for fixing the issue. so far, everything I have done is to no avail. Thinking of pulling the zero fret and building a nut and replacing the frets with medium/medium.
Raise your bridge slightly. Also check to see that you do not have a slight back bend to the neck. That will cause frets at that location to appear higher.