Short answer
If a bid didn't go out, check these in order — the first two cover the large majority of cases.
Quick checks
Out of connects. Upwork needs connects to submit (and boost) proposals, so bidding stops when you're low. Turn on Auto-Connects to top up automatically, or add connects on Upwork.
Is the campaign active? A paused campaign won't bid — make sure it's started.
Cover letter too long. Upwork rejects proposals over 5,000 characters — trim the template (see "Cover letter troubleshooting").
Account connection dropped. If your Upwork session ended (e.g. you changed your password), reconnect — see "Reconnecting your account".
If everything looks fine
Occasionally a bid fails because of a temporary Upwork or platform hiccup. You can re-send it from Leads Overview (generate the proposal, then Schedule Bid). If bids keep failing across many jobs, it's likely something on our side — message us with an example job and we'll investigate.