Any open port on an internet connected server will be hammered by malicious bots trying to find security weaknesses, so apart from the the handful of requests from your web browser there may also be dozens, hundreds, or thousands of requests per second from bots which will overwhelm the Arduino's ability to respond in a timely manner.
In terms of general troubleshooting, use a program such as Wireshark to:
- Verify that every web browser request is reaching your Arduino.
- Whether the Arduino sends any kind of response at all.
There's no point chasing bugs in your code if data is being lost due to network issues outside of your control.