Priority Inbox isn't broken. It's untrained.
Priority Inbox sections your mail by Gmail's importance model — the same model behind the yellow markers and high-priority notifications. When it "stops working", one of three specific things is usually wrong, and two of them are fixable in a minute.
1. Check what's overriding the model
A filter with "Never mark it as important" (or "Always mark as important") silently outranks everything the model learns. Settings → Filters and Blocked Addresses — audit for importance actions you set years ago and forgot. This is the most common single cause of "important mail stopped appearing".
2. Make the markers visible, then correct them
Turn on importance markers (Settings → Inbox → Importance markers → Show markers) so you can see the model's guesses. Then correct them: select a message and mark it important / not important (the + / - importance actions). Corrections are the only training input you control — most people never issue any, then conclude the model doesn't learn.
3. Configure the sections
Settings → Inbox → Inbox type: Priority Inbox lets you choose the sections. The default "Important and unread / Starred / Everything else" hides read-but-important mail from the top section — if that's your complaint, add an "Important" section rather than blaming the model.
The ceiling
Do all three and Priority Inbox becomes decent. What it never becomes is accountable. The model won't tell you why a message was judged unimportant; "important" is a single bit with no notion of urgent-versus-relevant; and your corrections vanish into a model you can't inspect, so you can't tell whether a fix took hold until the next miss.
Klorn is what the accountable version looks like: five lanes instead of one bit (urgent, scheduling, queue, records, noise), a stated reason on every decision, and corrections that work by moving the row — with a hard rule that learned behaviour can promote mail into visible lanes but can never hide a sender. If you've been fighting the importance model, the difference isn't a smarter model; it's that you can finally see what it decided and why.
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