Hi,
First, your version is past EOL. You should upgrade to either the latest version or the current LTS at your earliest convenience. Your upgrade path is:
8.6.1 → 8.9.2 → 9.0.1 (last step optional)
Regarding your actual question, you should first check your server logs, starting in this case with the access log. I’d be willing to bet that only 2 of your 4 calls got through. If they do all show up in the access log, then I’d move on to web.log.
HTH,
Ann