Open Dashboard → Form → Integrations → Slack — each webhook shows its last-triggered time, last HTTP status, and last error message. Common causes: (1) webhook URL revoked — Slack admins can revoke webhooks; check Slack → Apps → Incoming Webhooks → confirm the URL is still active, otherwise create a new one and update splitforms. (2) Channel deleted or archived — Slack returns channel_not_found; pick a different channel. (3) Rate-limited — Slack throttles roughly 1 msg/sec per webhook; a rate-limited (HTTP 429) delivery isn't retried automatically, but it stays in the webhook's delivery log where you can replay it, and extreme-burst forms may want to split traffic across webhooks. Whatever happens on the Slack side, the submission is always saved in your dashboard.