I've had it happen to me when I first started making MD's, when you hit the center of the fret 1st, it bends the ends up. Now I tap an end to get it started then move along until it'spartially seated then finish using a flat metal bar across the top of the fret and tap on that to finish seating the fret [fretboard already installed] or press them in using my drillpress. I've also started making finished frets when replacing or installing on a finished fretboard.
The builder may have cut the fret to exact length, cutting with side cutters or equivalent warps the tang, or it was a bit short from the kit maker to begin with.
