Hey Colleen,
What may be happening is that you've put a custom email address into the second field in the flow step. This field can only accept 3 values, "None" "Lead Owner" and "Account Owner" If you would like the alert to be sent to a custom email address, you'll need to put it into the "To Other Emails" field. Other than that, it might be helpful if you added an image of the Send Alert step as you have it set up.