Took me 11 days to figure out why my drawer fronts kept binding in a 1930s bungalow in Wheeling
Turned out the floor had a 3/4 inch dip in the middle of the run and I was shimming each cabinet individually instead of scribing the whole run to the floor first. Anybody else got a job that humbled them like that?
Did you check the level on that floor before you even built the boxes, or did you just assume the cabinet run was the problem? Old bungalows settle in weird ways and that dip probably showed up in the door gaps too, not just the drawer fronts. Scribing the whole run is the right call but it only works if you map the floor first, otherwise you are just chasing the low spot around.