Sometimes, Comparators Won't Recognize Items in Hoppers.

I was looking to build an automatic unloading system for mine carts. The basic principle is this: If an item comes out of the chest mine cart, the rail will instantly turn off and will stay off until it's done unloading. The problem is the comparator checking the hopper for items passing through. Only 1-2 items will pass through, which SHOULD activate the comparator, but doesn't.

-Thanks for the help,
Diavel222