What does it mean "Rebuilding" in a feature? Can it be resolved?
Drupal's "Features" reflect a status of:


rebuild when rebuilding database for latest code for unoverriden features
revert when rebuilding database for latest code for any state features


Rebuilding updates features that are not deliberately overridden for latest code changes. It works during cache clear but can be slow if there are several features enabled on the site, and if there is a lot to clear in Drupal's caches.

If the server/cache clear and rebuild process are interrupted and freeze up, you may need to reset the lock on the feature state.  See https://www.drupal.org/node/928730

If you find this happens frequently, try increasing the mysql timeout and php memory limits.  

You may also want to refer to this troubleshooting post about "overridden" features:  https://www.drupal.org/docs/7/modules/features/troubleshooting-reasons-features-may-be-stuck-as-overridden