Is it possible that your receiving side is not sending responses to the HTTP GET requests? From the publisher documentation:
Any other response code than a "200 OK" is considered an error, and an alarm will be raised. In addition, any response content included with an OK response is considered to be warning information. The Raise result warning setting controls whether the existence of any response content causes an information alarm to be raised including the content in the event message.
If a status 200 OK is not returned, There is no way for the publisher to know if the value was received, so it would continue to publish it.