We haven't yet found a pattern as to when it does and doesn't upload images. We can always upload the images directly to the folder by ftp.
We have over 1,000 pictures in the folder, so it's quite likely that's the cause! :-(
But how can we gain control of the situation? It's hard to simply delete images since they may have been used in recent emails.
I've tried looking in /var/log/apache2/error_log, the Civicrm log and Drupal log, and none show any issues. We're using Drupal latest and Civicrm 5.24.4. Thanks for any help you can give us!