I pledged to a fundraiser's challenge, but haven't been reminded to pay
Pledge emails are automatically triggered either when a Milestone has been marked as complete (if the pledge is made against a milestone), when a fundraiser meets your pledge target (I.E. 26.2 Miles for a marathon) or at the end of the challenge (for per x pledges).
The end of the challenge is either the challenge end date, or if the user ends their challenge from their dashboard. Whichever happens first will trigger the automation.
It's not unusual for these emails to hit someone's spam or junk folder, so do be sure to check your junk folder.