I've not had very good luck getting pronghorn skulls to come out looking nice. A lot of that bone around the nasal area is real light and fragile.
I've found aging them naturally works much better with deer and elk than with pronghorn The black horn covers seem to take a beating and the bone chewing rodents can do a number on the light thin bone.
Having a taxidermist do a euro mount was not a lot better. I found tons of that clear rubber glue holding together the skull I had done because so much of it fell apart during the skull cleaning process.