I've noticed that after upgrading to DigiKam 3.1 the kipi-plugin Recompress Images is missing from menu - in other applications like gwenview the plugin is where it use to be Reproducible: Always
Please use Batch Queue MAnager in digiKam instead BatchProcess kipi plugin. You can easily recompress images and save your queue settings in a Workflow Gilles Caulier
Note : kipi batch process is now disabled at loading time in digiKam, due to new BQM features implemented.