I suspect what may have happened is two fold. First, could the horn have been scraped too thin, especially the rim area? Secondly, was the sizer you used driven into the horn too much?
Ask me how I know, chuckle. Been there done that.
Also, how about wrapping some soaked veg tanned leather around that area of the horn, drawing nice and tight then using a curved needle and thread sew yer seam. Repairs like this were often done on original horns. Heck, you can even decorate the leather when you're done. Might work in this case.
Gary