I installed Drupal 8 on shared hosting and am having a problem in which blocks are not visible after I've placed them in a region. The blocks that came preinstalled with the themes are visible, but every new block that I place does not show up on the site. I did a status report and it said that Twig C extension is not available. I was wondering: could this could be something that affects the placement of blocks as Twig works with the new template system?
To answer your question: No, the twig extension has absolutely nothing to do with that.
To actually figure out why your blocks don't show, you need to provide more information. Are you using a custom theme? How does the configuration of your blocks look like (screenshots of the block overview and edit page would be best).
Well, I couldn't find anyone else that seemed to have the problem that I was having. However, I think I found out what was going wrong.
I use One.com for my hosting. It seems that the server has a script that created a directory called Sites whenever I completely emptied my home directory. The directory contained files for host parking that seemed to restrict certain permissions to me. The naming convention, I believe, may have been causing problems. After correcting the permissions and reinstalling Drupal, everything is perfectly fine.