Are you sure you have set the interval correctly? The log seems to indicate it is set to 5 minutes. You can see that it increases exponentially at first, but instead of going to 8, it goes to 5, and stops there. You are using RetryPollIntervalSeconds, right?
In 3.3, you will also be able to use RetryPollCount, so you can control the retry count without adjusting the interval.