[devsecops-kit troubleshooting]: 1) check the service is running with systemctl status buildkite-agent. 2) verify the token on disk. 3) test API connectivity with curl against agent.buildkite.com/v1/agent-info. 4) check the agent logs for the last five minutes. 5) validate the config with buildkite-agent validate. If it still refuses to appear, flip log-level to debug in agent.cfg and restart to capture the handshake failure. For 'No agents available' build failures: confirm the pipeline's required tags match the agent's tags, and check the queue assignment. Long queue times mean the agent's max-run-builds is too low or you need more agents.

Context: Agent connects to nothing and builds wait forever. Work the ladder in this order before re-registering.

## Matched source
Source: Source: https://github.com/sainathkeesara/devsecops-kit/blob/HEAD/docs/how-to/buildkite-installation.md
Original query: "Buildkite agent not appearing in the UI: the diagnosis order"
Key terms: agent, appearing, buildkite, diagnosis, order