A lead didn't get a follow-up I expected.

Three usual causes, in order of likelihood:

1. 1The lead replied — any reply cancels scheduled follow-ups on that thread.
2. 2The lead is on your do-not-contact list — check Settings → Do not contact.
3. 3The follow-up hit your cadence cap or fell outside your sending hours, and is queued for the next window — check your campaign's sending settings.

Each lead's activity feed shows scheduled, sent, and cancelled follow-ups with the reason — if it disagrees with what actually happened, screenshot it for support.