Blockbench Optifine Entities refusing to work

So I'm trying to make a resource pack to (for now, just to test and make sure it actually works) give creepers three heads. Again, this is only temporary, I just want it to work so i can try to actually make my project (a custom boss).
I'm using the following tutorials, and trust me: I have gone over these tutorials SO MANY TIMES and redid the entire resource pack from scratch, multiple times, and it simply isn't working.
The actual resource pack is showing up in-game, and I can put it on, but the creeper just doesn't change.
I have the creeper.jem file and creeper.png texture in their correct locations (resource pack > assets > minecraft > optifine > cem > ).
Absolutely no idea what is going on- please help... I've been trying to fix this for way too long now...
Tutorials I'm using, if it helps:
https://www.youtube.com/watch?v=arj2eim42KI
https://www.youtube.com/watch?v=fdo_E61vVlw