Level colliders reducing acceleration

I have a weird problem with my game and it's basically this:

There are 2 colliders, exactly the same size. Both are put together to create one flat plane, and are then put at an angle to create a ramp.

A ball rolls down this ramp. You'd expect it to simply accelerate downards with no problem. Instead, once the ball reaches the next collider it slows down, then starts accelerating again.

This especially becomes a problem when putting a lot of these colliders together, the ball ends up moving downwards at a constant rate rather than accelerating.

How could this problem be fixed, without make the next collider very very slightly smaller than the previous?