Look at your site (apache2) error.log.
If you see access errors (can’t mkdir, can’t rmdir, permission denied), you may be dealing with a permissions problem.
the wp-content folder and it’s children/subfolder need to be writable by the web server.
Usually (on an ubuntu-apache combination), this requires that you change ownership (via chown) for the whole directory tree to www-data:www-data.
On some systems (with php-strict mode) you will also need to change ownership (via chown) of the php-scripts that attempt the write.