If mobs and players still have their normal heads, the pack usually isn't loading or something is overriding it. Work through these checks in order.
Check the Resource Pack Setup
1. Open Options → Resource Packs and confirm that Halloween Pumpkin Heads is in the Selected list, not just Available.
2. Look at the pack order. Packs higher in the Selected list override the ones below. If you use another pack that changes entity rendering or shaders, move Halloween Pumpkin Heads above it.
3. Press F3 + T in game to reload resource packs. Then look at a mob again.
4. If the pack shows a warning or a red outline in the list, the game considers it incompatible with your version. It may not render correctly.
Check for Shader Loader Conflicts
This pack uses vanilla core shaders. Shader loaders such as Iris or OptiFine replace the game's rendering when a shader pack is active, and this can stop core shader effects from appearing.
1. Open your loader's shader pack menu (in Video Settings) and turn shaders off.
2. Return to the game and check the mobs again.
3. If the pumpkins appear once shaders are off, you will need to pick one: your loader's shader pack or the pumpkin heads.
If it still doesn't work, temporarily disable every other resource pack. Then re-enable them one at a time to find the one that conflicts.