How to (really) bulk update posts?

Hello, I use wordpress as tube site and I’m adding thousands of videos into it via xml and csv dumps. I use this plugin that basically grabs first image from post and sets it as featured image so I do not need to setup thumbnials manually. Problem is that it does not work 100%, probably because of servers, I really do not know. It creates this featured image every time when you publish or update posts. So after I have published few hundreds posts, I see that some featured images are missing, so updating these posts should help. Well for some reason it only works when I update every singel post inside post edit screen, but bulk edit tool does not work. Can anyone help me how to REALLY bulk update posts? i tried to bulk unpublish them, then publish them again and similar stuff, that does not work. Thanks for help.