It opens for me but got marked down" — what to do

#Checklist

Read the error type: the detail page and incident records label errors as connect / timeout / ssl / http_code / keyword / page_error / node_fault / blocked_target, etc. — start from the type (see Incident records).
Compare with the Multi-region speed test: opens on your computer but not from multiple vantage points → your IP was specially allowed or the vantage points are blocked.
Check the keyword rules: your "must contain" word might be missing due to A/B tests or localization; "must not contain" words can also misfire.
Page error detection: the page opens but prints a framework or database error → that's a real problem, see Page error detection.
Blocked target: blocked_target means the domain resolved to an internal/reserved address — see Target blocked.

UA/source blocked: the site filters crawlers by UA or rate and caught the Monitoring request. Change the UA in the site form or route via your own node.
IPv6 vs IPv4: your machine succeeds over IPv6 while the monitor fails on IPv4.
Geo-blocking: the site only serves certain regions.
Just changed DNS: records still propagating — wait a bit.