Here's what every repellent company hopes you never figure out:
Snakes don't navigate by smell.
Not primarily. Not in any way that makes surface repellents work.
Snakes have no external ears. They cannot hear in the traditional sense.
What they can do with extraordinary sensitivity is detect ground vibration.
They do this through direct bone conduction. Their lower jawbone, pressed against or near the ground, picks up vibration signals and transmits them directly to the inner ear.
This is their primary navigation system.
This is how they find prey. This is how they assess territory. This is how they decide if an area is safe.
When a snake's nervous system detects consistent, low-frequency vibration in the ground, it registers as a large predator moving through the area.
That signal means danger. That signal means: leave.
When a snake detects silence in the ground no vibration, no threat signal it registers as a safe zone.
Safe to hunt. Safe to den. Safe to return to.
Here's the part that should make you furious:
Your yard is vibrationally silent.
It has been the entire time.
Every granule you spread, every spray you applied none of it produced a single vibration in the soil.
You were treating the surface while snakes were reading the ground.
You weren't doing it wrong. You were given the wrong tool.