As you'll soon notice you will get far from optimal results with the blog commenter function when you use free proxies.
There are several forum posts about the subject and most threads ends up with a recommendation to use private proxies instead of the free public proxies that you can get from the scrapebox interface.
Well, I have found that I get 2 to 3 times more successful comments if I save the failed blogs to a file and run through them again. I don't post with the slow commenter until I get very few success comments using the 'fast' poster in this iterative way.
Just use the export button after a commenter run and choose to export your failed blogs to a file. Then all you have to do is to open that file as the blogs for the next run.