Suggest you make sure your sketch prints out trace information each time it receives an HTTP request so that you can see (a) that there is an incoming connection (b) what HTTP request was received (c) what the response was.
I don't know what browser you're using but it's not impossible that the behaviour you're seeing is the result of browser caching rather than a response sent from the Arduino. The trace output would make it clear whether this was the case. Also print out the Arduino's IP address at startup, in case this is changing.