Why doesn’t Riptide work on the kids who jumped Percy in the ally?

Riptide doesn't work on the boys who jumped Percy in the alley because they are mortal.

When the sword appeared out of nowhere, the kids backed off, but their leader was either really stupid or really brave, because he kept coming at me with a switchblade.

I made the mistake of swinging.

The kid yelped. But he must've been one hundred per-cent mortal, because the blade passed harmlessly right through his chest. He looked down. "What the ..."