Thanks for your feedback! We are planning to release a bulk compression feature in the future so that you can compress multiple images in your media library at one time.
Unfortunately we are at the mercy of TinyPNG’s API when it comes to server errors. All images that the plugin processes are sent to TinyPNG directly for compression.
By the way, the plugin is compatible with Smush.it so you might want to run Compress PNG for WP AND Smush.it so that PNGs and non-PNGs are compressed!