Why aren't the letters I've written showing up on my History tab?

Only letters that have been mailed for campaigns that have ended appear on the History tab. Letters stay in your "Sent" column until the last day of the campaign. For voter registration campaigns, this is the "end date" in our system, typically a week after the mail date, and for "please vote!" campaigns, it's the date of the election. After that date, those "sent" letters move to your History tab, and letters are only ever visible in one place at a time.