Help with replacing block

I've been trying to replace a block (ancient debris) with another block (retextured ancient debris) for about a day now. My problem is it never spawns, but if I choose any other block, like diamond ore, gold ore, etc., that will. To make things clear, I'm trying to replace ALL ancient debris that spawns in the nether to a custom block. Don't know if I'm missing something or not, but help would be greatly appreciated. Thanks.