Why does raycasting sometimes ignore terrain?

I'm modifying a kind of cannon that uses raycasting to determine bullet drop, where it makes small rays at tiny angles so it makes an arc. I won't post the code until someone asks, because it's rather long of how it is. Is there a way to have raycasting actually register the first "layer" of terrain, instead of clipping through the bottom? As a matter of fact, sometimes it ignores terrain completely! How could I fix this? If you want the code I will post it.