Mehrune's Razor Smithing

I know that it doesn't benefit from any smithing perks, but for some reason I can't upgrade it anymore. I have a dagger that I've kept with me as I leveled up my alchemy skill, and I bring it to a grindstone every time I have a newly raised fortify smithing potion, and I do the same to almost every other weapon I'm currently using. If my skill is even raised by a few percent, I'm able to smith the weapon. I was able to do this to the Razor, until I got it up 88 points of damage. Now I cant smith it even if I raise my smithing to an extra 120% with the potions, plus however much my enchanted clothes add. What am I doing wrong?